Hydrocarbon adsorbent state monitoring device

ABSTRACT

A characteristic change of a humidity sensor  19  is detected based on the transitional characteristics of output data of the humidity sensor  19  with respect to integrated moisture quantity data representative of an integrated amount of moisture that is supplied to an HG adsorbent  7  by an exhaust gas after an engine  1  has started to operate. A parameter for grasping a state of the HC adsorbent  7  is corrected. The state, such as a deteriorated state or the like, of a hydrocarbon adsorbent (HC adsorbent) is properly monitored while compensating for the effect of the characteristic change of the humidity sensor and the effect of variations of the characteristics of individual units of the humidity sensor.

TECHNICAL FIELD

The present invention relates to an apparatus for monitoring the stateof a hydrocarbon adsorbent disposed in the exhaust passage of aninternal combustion engine.

BACKGROUND ART

Some known systems for purifying exhaust gases emitted from internalcombustion engines have an exhaust gas purifier disposed in the exhaustpassage, which may comprise a hydrocarbon adsorbent such as zeolite or ahydrocarbon adsorbing catalyst comprising a composite combination of ahydrocarbon adsorbent and a three-way catalyst, for adsorbinghydrocarbons (HC) in the exhaust gas while the catalytic converter suchas a three-way catalyst or the like is not functioning sufficiently,i.e., while the catalytic converter is not sufficiently activated aswhen the internal combustion engine starts to operate at a lowtemperature. The hydrocarbon adsorbent has a function to adsorbhydrocarbons in the exhaust gas at relatively low temperatures below100° C., for example, and operates to release the adsorbed hydrocarbonswhen heated to a certain temperature in the range from 100 to 250° C.,for example.

The applicant of the present application has proposed the followingtechnique of monitoring a state, e.g., a deteriorated state, of ahydrocarbon adsorbent of the type described above: The inventors of thepresent invention have found that the hydrocarbon adsorbent is capableof adsorbing not only hydrocarbons contained in exhaust gases, but alsomoisture contained in exhaust gases. The ability of the hydrocarbonadsorbent to adsorb moisture, i.e., the maximum amount of moisture thatcan be adsorbed by the hydrocarbon adsorbent, is highly correlated tothe ability of the hydrocarbon adsorbent to adsorb hydrocarbons, i.e.,the maximum amount of hydrocarbons that can be adsorbed by thehydrocarbon adsorbent. As the hydrocarbon adsorbent progressivelydeteriorates, both the ability to adsorb moisture and the ability toadsorb hydrocarbons are progressively lowered in the same manner.Therefore, when the ability of the hydrocarbon adsorbent to adsorbmoisture is evaluated, the ability of the hydrocarbon adsorbent toadsorb hydrocarbons can also be evaluated based on the evaluated abilityto adsorb moisture. According to the technique proposed by theapplicant, a humidity sensor is disposed downstream of the hydrocarbonadsorbent. Using output data of the humidity sensor, the ability of thehydrocarbon adsorbent to adsorb moisture and hence the ability of thehydrocarbon adsorbent to adsorb hydrocarbons are grasped, thusmonitoring the deteriorated state of the hydrocarbon adsorbent.

For adequately evaluating the deteriorated state of the hydrocarbonadsorbent using output data from the humidity sensor in various manyenvironments that the internal combustion engine is subject to while inoperation, the humidity sensor that is exposed to high-temperatureexhaust gases is required to be highly durable over a long period oftime, and also required to have minimum changes in the agingcharacteristics thereof and to suffer minimum characteristic variationsamong individual units of humidity sensors.

However, it is generally difficult for humidity sensors to fully satisfyall the above requirements. It would need a large expenditure of moneyand labor to develop humidity sensors that fully satisfy all the aboverequirements, and such humidity sensors would be highly expensive tomanufacture.

The present invention has been made in view of the above background. Itis an object of the present invention to provide a hydrocarbon adsorbentstate monitoring apparatus for adequately monitoring the deterioratedstate or the like of a hydrocarbon adsorbent by compensating for effectsof changes in the characteristics of humidity sensors that are used andalso compensating for effects of characteristic variations amongindividual units of the humidity sensors.

DISCLOSURE OF THE INVENTION

To achieve the above object, there is provided in accordance with thepresent invention an apparatus for monitoring a state of a hydrocarbonadsorbent disposed in an exhaust passage of an internal combustionengine for adsorbing hydrocarbons in an exhaust gas emitted from theinternal combustion engine, using output data of a humidity sensordisposed near the hydrocarbon adsorbent, characterized by characteristicchange detecting means for detecting a characteristic change of thehumidity sensor based on output data of the humidity sensor under apredetermined condition, and characteristic change compensating meansfor correcting a parameter to grasp the state of the hydrocarbonadsorbent using the output data of the humidity sensor, based on thecharacteristic change detected by the characteristic change detectingmeans (first invention).

According to the above invention, the parameter to grasp the state ofthe hydrocarbon adsorbent is corrected based on the characteristicchange of the humidity sensor which is detected based on the output dataof the humidity sensor under a certain condition (e.g., a condition withrespect to a timing to acquire the output data of the humidity sensor,an operating state of the internal combustion engine, or the like). Itis thus possible to obtain a parameter where the effect of thecharacteristic change of the humidity sensor has been compensated for.Using such a parameter, the state of the hydrocarbon adsorbent (adeteriorated state of the hydrocarbon adsorbent, a state in whichmoisture is adsorbed by the hydrocarbon adsorbent, or the like) canappropriately be grasped while compensating for the effect of thecharacteristic change of the humidity sensor. As the effect of thecharacteristic change of the humidity sensor can be compensated for, therequirements for the steadiness of the characteristics of the humiditysensor are lessened, and the humidity sensor is allowed to have certaincharacteristic changes. Therefore, the costs needed to develop andmanufacture the humidity sensor can be reduced.

According to the present invention, the parameter may be a parameterrepresentative of the state of the hydrocarbon adsorbent to be grasped,a parameter such as a threshold to be compared with such a parameter, oran intermediate parameter for use in a process for determining suchparameters. The characteristic change of the humidity sensor that isdetected by the characteristic change detecting means may, for example,be a characteristic change of the humidity sensor with respect todesired reference characteristics, such as characteristics of abrand-new humidity sensor.

According to the present invention (first invention), the apparatus hasintegrated moisture quantity data generating means for sequentiallygenerating data of an integrated amount of moisture supplied to thehydrocarbon adsorbent by the exhaust gas emitted from the internalcombustion engine after the internal combustion engine has started tooperate, wherein the characteristic change detecting means detects thecharacteristic change of the humidity sensor based on a change due tothe characteristic change of the humidity sensor, of changes oftransitional characteristics of the output data of the humidity sensorwith respect to the data generated by the integrated moisture quantitydata generating means after the internal combustion engine has startedto operate (second invention).

Alternatively, the characteristic change detecting means may detect thecharacteristic change of the humidity sensor based on a change due tothe characteristic change of the humidity sensor, of changes oftransitional characteristics of the output data of the humidity sensorwith respect to a period of time that has elapsed after the internalcombustion engine has started to operate (third invention).

Specifically, as described in detail later on, the inventors have foundthat the output data of the humidity sensor near the hydrocarbonadsorbent exhibits a characteristic transition with respect to the data(the data generated by the integrated moisture quantity data generatingmeans) of the integrated amount of moisture supplied to the hydrocarbonadsorbent by the exhaust gas emitted from the internal combustion engineafter the internal combustion engine has started to operate or a periodof time that has elapsed after the internal combustion engine hasstarted to operate. The transitional characteristics change depending onthe characteristic change of the humidity sensor. The transitionalcharacteristics include a portion which changes due to not only thecharacteristic change of the humidity sensor, but also the effect of thestate in which the hydrocarbon adsorbent adsorbs moisture, and alsoinclude a portion which changes due to only the characteristic change ofthe humidity sensor. Based on a change due to the characteristic changeof the humidity sensor, of changes of the transitional characteristics,therefore, the characteristic change of the humidity sensor canappropriately be detected. Since the transitional characteristics arealso subject to the effect of the state in which the hydrocarbonadsorbent adsorbs moisture, it is also possible to grasp the state inwhich the hydrocarbon adsorbent adsorbs moisture based on thetransitional characteristics while grasping the characteristic change ofthe humidity sensor.

The data representative of the integrated amount of moisture may be dataof the integrated amount of moisture itself, but may basically be datasubstantially proportional to the integrated amount of moisture. Forexample, the integrated value of an amount of fuel or an amount ofintake air supplied to the internal combustion engine after the internalcombustion engine has started to operate may be used as the datarepresentative of the integrated amount of moisture. If the operatingstate of the internal combustion engine after the internal combustionengine has started to operate is a substantially constant operatingstate, e.g., an idling state, then the period of time that has elapsedafter the internal combustion engine has started to operate may be usedas the data representative of the integrated amount of moisture.

According to the present invention (second invention) wherein thecharacteristic change of the humidity sensor is detected based on achange of the transitional characteristics of the output data of thehumidity sensor with respect to the data representative of theintegrated amount of moisture after the internal combustion engine hasstarted to operate, more specifically, the characteristic changedetecting means may detect the characteristic change of the humiditysensor based on a change from a predetermined reference value ofcharacteristic change detecting output data which comprises the outputdata of the humidity sensor at the time when the data generated by theintegrated moisture quantity data generating means has reached apredetermined value after the internal combustion engine has started tooperate (fourth invention). Similarly, according to the presentinvention (third invention) wherein the characteristic change of thehumidity sensor is detected based on a change of the transitionalcharacteristics of the output data of the humidity sensor with respectto the period of time that has elapsed after the internal combustionengine has started to operate, more specifically, the characteristicchange detecting means detects the characteristic change of the humiditysensor based on a change from a predetermined reference value ofcharacteristic change detecting output data which comprises the outputdata of the humidity sensor at the time when the period of time that haselapsed after the internal combustion engine has started to operate hasreached a predetermined value (fifth invention).

Specifically, as described in detail later on, the inventors have foundthat if the humidity sensor is disposed downstream of the hydrocarbonadsorbent, the humidity at the location of the humidity sensorimmediately after the internal combustion engine has started to operateis of a substantially constant low humidity level due to the adsorptionof moisture in the exhaust gas by the hydrocarbon adsorbent. When theintegrated amount of moisture or the elapsed period of time increasesuntil the adsorption of moisture by the hydrocarbon adsorbent issaturated, the humidity at the location of the humidity sensor increasesmonotonously from a low humidity level to a high humidity level untilfinally it reaches the humidity inherent in the exhaust gas (thehumidity of the exhaust gas when the hydrocarbon adsorbent or the likedoes not adsorb moisture). The output data of the humidity sensor at thetime when the humidity at the location of the humidity sensor reachesthe humidity inherent in the exhaust gas is of a substantially constantvalue. When the humidity sensor suffers a characteristic change due toits deterioration, a timing at which the output data of the humiditysensor becomes substantially constant or the substantially constantlevel changes depending on the characteristic change.

If the humidity sensor is disposed upstream of the hydrocarbonadsorbent, then since the region near the hydrocarbon adsorbent is dryimmediately after the internal combustion engine has started to operate,the humidity at the location of the humidity sensor is of a low humiditylevel for a relatively short period of time, and then changes quickly toa high humidity level and reaches the humidity inherent in the exhaustgas. The output data of the humidity sensor at the time when thehumidity at the location of the humidity sensor reaches the humidityinherent in the exhaust gas is of a substantially constant value, aswith the above case. When the humidity sensor suffers a characteristicchange due to its deterioration, a timing at which the output data ofthe humidity sensor becomes substantially constant or the substantiallyconstant level changes depending on the characteristic change.

According to the present invention, the output data of the humiditysensor at the time when the data generated by the integrated moisturequantity data generating means (the data representative of theintegrated amount of moisture) has reached a predetermined value or thetime when the period of time that has elapsed after the internalcombustion engine has started to operate has reached a predeterminedvalue is used as the characteristic change detecting output data. Thepredetermined value may be determined such that the humidity at thelocation of the humidity sensor reaches the inherent humidity of theexhaust gas or a humidity close thereto when the data representative ofthe integrated amount of moisture or the elapsed period of time hasreached the predetermined value. According to the present invention, thecharacteristic change of the humidity sensor is detected based on achange from the reference value of the characteristic change detectingoutput data. Thus, the characteristic change of the humidity sensor canappropriately be detected.

According to the present invention (second invention or fourthinvention) wherein the characteristic change of the humidity sensor isdetected based on a change of the transitional characteristics of theoutput data of the humidity sensor with respect to the datarepresentative of the integrated amount of moisture after the internalcombustion engine has started to operate, if the state of thehydrocarbon adsorbent to be monitored comprises a deteriorated state ofthe hydrocarbon adsorbent, then the humidity sensor is disposeddownstream of the hydrocarbon adsorbent. A changing timing at which ahumidity represented by the output data of the humidity sensor changesto a tendency to increase monotonously from a low humidity level to ahigh humidity level after the internal combustion engine has started tooperate is detected, and the data generated by the integrated moisturequantity data generating means at the detected changing timing is usedas the parameter for grasping the deteriorated state of the hydrocarbonadsorbent (sixth invention).

Similarly, according to the present invention (third invention or fifthinvention) wherein the characteristic change of the humidity sensor isdetected based on a change of the transitional characteristics of theoutput data of the humidity sensor with respect to the period of timethat has elapsed after the internal combustion engine has started tooperate, if the state of the hydrocarbon adsorbent to be monitoredcomprises a deteriorated state of the hydrocarbon adsorbent, thehumidity sensor is disposed downstream of the hydrocarbon adsorbent. Theapparatus further comprises integrated moisture quantity data generatingmeans for sequentially generating data of an integrated amount ofmoisture supplied to the hydrocarbon adsorbent by the exhaust gasemitted from the internal combustion engine after the internalcombustion engine has started to operate, and changing timing detectingmeans for detecting a changing timing at which a humidity represented bythe output data of the humidity sensor changes to a tendency to increasemonotonously from a low humidity level to a high humidity level afterthe internal combustion engine has started to operate. The datagenerated by the integrated moisture quantity data generating means atthe changing timing detected by the changing timing detecting means isused as the parameter for grasping the deteriorated state of thehydrocarbon adsorbent (seventh invention).

Specifically, as described above, if the humidity sensor is disposeddownstream of the hydrocarbon adsorbent, then the humidity at thelocation of the humidity sensor immediately after the internalcombustion engine has started to operate is of a substantially constantlow humidity level due to the adsorption of moisture in the exhaust gasby the hydrocarbon adsorbent. When the adsorption of moisture by thehydrocarbon adsorbent is saturated, the humidity at the location of thehumidity sensor changes to a tendency to increase monotonously from alow humidity level to a high humidity level. Therefore, a changingtiming at which the humidity represented by the output of the humiditysensor changes to a tendency to increase monotonously from a lowhumidity level to a high humidity level signifies a timing at which theadsorption of moisture by the hydrocarbon adsorbent is saturated (atthis timing, the adsorption of moisture by the hydrocarbon adsorbent issaturated, the timing may also be referred to as “absorption saturationtiming”). The data generated by the integrated moisture quantity datagenerating means at the absorption saturation timing (data of theintegrated amount of moisture) corresponds to a maximum amount ofmoisture and hydrocarbons that can be adsorbed by the hydrocarbonadsorbent, and represents a deteriorated state of the hydrocarbonadsorbent. In this case, the absorption saturation timing (changingtiming) at which the humidity represented by the output of the humiditysensor changes to a tendency to increase monotonously from a lowhumidity level to a high humidity level is generally affected by notonly the deteriorated state of the hydrocarbon adsorbent, but also theeffect of the characteristic change of the humidity sensor. The effectof the characteristic change of the humidity sensor can be compensatedfor by correcting the data generated by the integrated moisture quantitydata generating means at the absorption saturation timing depending onthe characteristic change of the humidity sensor detected by thecharacteristic change detecting means. Therefore, by using the datagenerated by the integrated moisture quantity data generating means atthe absorption saturation timing as the parameter for grasping thedeteriorated state of the hydrocarbon adsorbent, the deteriorated stateof the hydrocarbon adsorbent can appropriately be grasped whilecompensating for the effect of the characteristic change of the humiditysensor.

According to either one of the first through fifth inventions, if thestate of the hydrocarbon adsorbent to be monitored comprises adeteriorated state of the hydrocarbon adsorbent, then the output data ofthe humidity sensor before a humidity represented by the output data ofthe humidity sensor is converged to a humidity outside of the exhaustpassage after the internal combustion engine has stopped operating maybe used as the parameter for grasping the deteriorated state of thehydrocarbon adsorbent (eighth invention).

Specifically, as described in detail later on, the inventors have foundthat the humidity near the hydrocarbon adsorbent and hence the outputdata of the humidity sensor after the internal combustion engine hasstopped operating exhibit the tendency of a characteristic transitionwith respect to the deteriorated state of the hydrocarbon adsorbent. Forexample, when the temperature of the hydrocarbon adsorbent drops afterthe internal combustion engine has stopped operating, the hydrocarbonadsorbent adsorbs moisture in the exhaust gas around (near) thehydrocarbon adsorbent. When the adsorption of moisture is saturated, thehumidity near the hydrocarbon adsorbent and hence the output data of thehumidity sensor are kept at a substantially constant level for arelatively long period of time (the humidity suffers very smalltime-dependent changes). The amount of moisture that can be adsorbed bythe hydrocarbon adsorbent is smaller as the hydrocarbon adsorbent ismore deteriorated, so that the above constant level depends on thedeteriorated state of the hydrocarbon adsorbent. When a sufficientlylong period of time elapses after the internal combustion engine hasstopped operating, since a gas exchange between the interior of theexhaust pipe including the region around the hydrocarbon adsorbent andthe external atmosphere progresses, the humidity near the hydrocarbonadsorbent is finally converged to the humidity outside of the exhaustpassage.

As described above, because the humidity near the hydrocarbon adsorbentexhibits a characteristic transition with respect to the deterioratedstate of the hydrocarbon adsorbent after the internal combustion enginehas stopped operating, the output data of the humidity sensor after theinternal combustion engine has stopped operating and before the humiditynear the hydrocarbon adsorbent is finally converged to the humidityoutside of the exhaust passage can be used as the parameter to grasp thedeteriorated state of the hydrocarbon adsorbent. Simultaneously, bycorrecting the parameter depending on the characteristic change of thehumidity sensor, the deteriorated state of the hydrocarbon adsorbent canappropriately be grasped while compensating for the effect of thecharacteristic change of the humidity sensor.

According to the present invention (eighth invention) wherein the outputdata of the humidity sensor after the internal combustion engine hasstopped operating is used as the parameter, it is preferable to use theoutput data of the humidity sensor in a period of time in which thehumidity represented by the output of the humidity sensor is maintainedat a substantially constant level after the internal combustion enginehas stopped operating, as the parameter to grasp the deteriorated stateof the hydrocarbon adsorbent. Specifically, as described above, theoutput data (output data having a substantially constant level) of thehumidity sensor in the state wherein the humidity near the hydrocarbonadsorbent is kept substantially constant after the internal combustionengine has stopped operating depends on the deteriorated state of thehydrocarbon adsorbent. In the state wherein the humidity is keptconstant, since the output data of the humidity sensor is also stable,the output data of the humidity sensor is highly reliable as dependingon the deteriorated state of the hydrocarbon adsorbent. Therefore, thedeteriorated state of the hydrocarbon adsorbent can be graspedaccurately. Furthermore, inasmuch as the humidity sensor may be of atype capable of detecting a substantially constant humidity, it is notrequired to be highly responsive, but may comprise a relativelyinexpensive sensor.

According to the present invention (eighth invention or ninth invention)wherein the output data of the humidity sensor after the internalcombustion engine has stopped operating is used as the parameter, thedeteriorated state of the hydrocarbon adsorbent is preferably graspedbased oh the parameter after the internal combustion engine has stoppedoperating under a predetermined condition (tenth invention). With thisarrangement, an exhaust gas state (the humidity state of the exhaust gasor the like) in the exhaust passage after the internal combustion enginehas stopped operating can be maintained in an optimum state forevaluating the deteriorated state of the hydrocarbon adsorbent.Therefore, the reliability is increased in grasping the deterioratedstate of the hydrocarbon adsorbent using the output data of the humiditysensor after the internal combustion engine has stopped operating as theparameter.

More specifically, the predetermined condition referred to aboveincludes a condition relative to an air-fuel ratio before the internalcombustion engine stops operating. The deteriorated state of thehydrocarbon adsorbent should preferably be grasped based on theparameter if the air-fuel ratio immediately before the internalcombustion engine stops operating is continuously maintained at a levelnear the stoichiometric air-fuel ratio for a predetermined period oftime or longer.

Specifically, if the internal combustion engine stops operating isoperated with its air-fuel ratio (the air-fuel ratio of an air-fuelmixture to be combusted by the internal combustion engine) maintained ata level near the stoichiometric air-fuel ratio, the exhaust gas emittedfrom the internal combustion engine contains relatively much moistureand the concentration of moisture in the exhaust gas is substantiallyconstant. Therefore, if the air-fuel ratio immediately before theinternal combustion engine stops operating is continuously maintained ata level near the stoichiometric air-fuel ratio for a predeterminedperiod of time or longer, then an exhaust gas containing sufficientmoisture at a substantially constant concentration is present in thevicinity of the hydrocarbon adsorbent immediately after the combustionengine stops operating. Consequently, the hydrocarbon adsorbent smoothlyadsorbs moisture and is saturated, and the transition of the moisture inthe vicinity of the hydrocarbon adsorbent distinctly depends on thedeteriorated state of the hydrocarbon adsorbent.

If the above predetermined condition includes a condition relative to awarmed-up state of the internal combustion engine before it stopsoperating, and the engine temperature immediately before the internalcombustion engine stops operating is equal to or higher than apredetermined temperature, then the deteriorated state of thehydrocarbon adsorbent should preferably be evaluated based on the outputof the humidity sensor. With this arrangement, when the internalcombustion engine stops operating while it has sufficiently been warmedup, i.e., while the combustion of the air-fuel mixture in the internalcombustion engine has been stabilized, and the hydrocarbon adsorbent hassufficiently been heated to release the moisture adsorbed thereby, thedeteriorated state of the hydrocarbon adsorbent is grasped based on theoutput data (the parameter) of the humidity sensor after the internalcombustion engine has stopped operating. Therefore, variations in thehumidity of the exhaust gas at the hydrocarbon adsorbent immediatelyafter the internal combustion engine has stopped operating are reduced,and after the temperature of the hydrocarbon adsorbent has dropped to acertain extent, the hydrocarbon adsorbent can smoothly adsorb a maximumamount of moisture depending on the deteriorated state of thehydrocarbon adsorbent. As a result, the transition of the output of thehumidity sensor after the internal combustion engine has stoppedoperating becomes more reliable as corresponding to the deterioratedstate of the hydrocarbon adsorbent, and the accuracy of the deterioratedstate of the hydrocarbon adsorbent as grasped using the parameter isincreased.

According to the present invention (first through tenth inventions), thecharacteristic change compensating means should preferably correct theparameter when the characteristic change of the humidity sensor which isdetected by the characteristic change detecting means exceeds apredetermined quantity (eleventh invention). With this arrangement, whenthe parameter does not need to be corrected as when a characteristicchange of the humidity sensor is detected due to a temporarydisturbance, the parameter is prevented from being corrected.

In either one of the first through eleventh inventions, the deterioratedstate of the hydrocarbon adsorbent should preferably be prohibited frombeing grasped based on the parameter when the characteristic change ofthe humidity sensor which is detected by the characteristic changedetecting means exceeds a predetermined upper limit quantity (twelfthinvention). Specifically, if the humidity sensor suffers an excessivecharacteristic change, the humidity sensor may possibly be excessivelydeteriorated or may possibly suffer a failure. In such a situation, itis difficult to obtain the parameter capable of appropriately graspingthe state of the hydrocarbon adsorbent. In this case, therefore, thestate of the hydrocarbon adsorbent is prohibited from being graspedbased on the parameter, and hence is prevented from being grasped inerror.

According to the present invention (first through twelfth inventions),the humidity sensor preferably has characteristic data holding means forholding, in advance, data of characteristics of an individual unit ofthe humidity sensor, and the characteristic change detecting meanspreferably detects the characteristic change of the humidity sensorbased on the output data of the humidity sensor and the data held by thecharacteristic data holding means (thirteenth invention).

With the above arrangement, since the characteristic change of thehumidity sensor is detected using the data held by the characteristicdata holding means associated with an individual unit of the humiditysensor, i.e., the data of the characteristics of an individual unit ofthe humidity sensor, it is possible to compensate for not only theeffect of the characteristic change of the humidity sensor, but also theeffect of characteristic variations of individual units of the humiditysensor in grasping the state of the hydrocarbon adsorbent in graspingthe deteriorated state of the hydrocarbon adsorbent. As a result, therequirements for the uniformity of the characteristics of individualunits of the humidity sensor are lessened, and the costs needed todevelop and manufacture the humidity sensor can be reduced.

If the characteristic change of the humidity sensor is detected based onthe transitional characteristics of the output data of the humiditysensor with respect to the integrated amount of moisture after theinternal combustion engine has started to operate (fourth invention),then the humidity sensor preferably has characteristic data holdingmeans for holding, in advance, data specifying the predetermined valuerelative to the data generated by the integrated moisture quantity datagenerating means, as the data of the characteristics of the individualunit of the humidity sensor. The characteristic change detecting meanspreferably acquires the characteristic change detecting output data ofthe humidity sensor using the predetermined value which is specified bythe data held by the characteristic data holding means (fourteenthinvention).

With the above arrangement, the predetermined value (the predeterminedvalue relative to the data representing the integrated amount ofmoisture) specifying the timing at which the characteristic changedetecting output data of the humidity sensor is acquired is adjusted soas to match the characteristics of the individual unit of the humiditysensor. As a consequence, the effect of characteristic variations ofindividual units of the humidity sensor can appropriately be compensatedfor.

If the characteristic change of the humidity sensor is detected based onthe transitional characteristics of the output data of the humiditysensor with respect to the integrated amount of moisture after theinternal combustion engine has started to operate or if thecharacteristic change of the humidity sensor is detected based on thetransitional characteristics with respect to the elapsed period of time(fourth or fifth invention), then the humidity sensor preferably hascharacteristic data holding means for holding, in advance, dataspecifying the reference value relative to the characteristic changedetecting output data as the data of the characteristics of theindividual unit of the humidity sensor, and the characteristic changedetecting means preferably acquires the characteristic change detectingoutput data of the humidity sensor using the reference value which isspecified by the data held by the characteristic data holding means(fifteenth invention).

With the above arrangement, the reference value serving as a referencefor detecting the characteristic change of the humidity sensor isadjusted so as to match the characteristics of the individual unit ofthe humidity sensor. As a consequence, the characteristic change of thehumidity sensor can be detected while appropriately compensating for theeffect of characteristic variations of individual units of the humiditysensor.

According to the present invention (thirteenth through fifteenthinventions) wherein the effect of characteristic variations ofindividual units of the humidity sensor is compensated for, thecharacteristic data holding means preferably comprises a resistiveelement having a resistance depending on the value of the data of thecharacteristics of the individual unit of the humidity sensor (sixteenthinvention).

With the above arrangement, the characteristic data holding means can beof an inexpensive and simple structure. Since the resistance of theresistive element can be detected relatively easily, the data of thecharacteristics of the individual unit of the humidity sensor can easilybe recognized. The characteristic data holding means should preferablybe associated with a connector by which the humidity sensor is connectedto an electronic circuit unit or the like which processes the outputdata of the humidity sensor, for example.

The humidity sensor used for monitoring the state of the hydrocarbonadsorbent is not limited to a single humidity sensor, but may comprise aplurality of humidity sensors. If such a plurality of humidity sensorsare employed, then the effects of characteristic changes of therespective humidity sensors should preferably be compensated for.

Consequently, according to another aspect of the present invention,there is also provided an apparatus for monitoring a state of ahydrocarbon adsorbent disposed in an exhaust passage of an internalcombustion engine for adsorbing hydrocarbons in an exhaust gas emittedfrom the internal combustion engine, using output data of a plurality ofhumidity sensors disposed at different locations near the hydrocarbonadsorbent, characterized by characteristic change detecting means fordetecting characteristic changes of the humidity sensors based on outputdata of the respective humidity sensors under a predetermined condition,and characteristic change compensating means for correcting a parameterto grasp the state of the hydrocarbon adsorbent using the output data ofthe humidity sensors, based on the characteristic changes of thehumidity sensors detected by the characteristic change detecting means(seventeenth invention).

According to the above invention (seventeenth invention), the parameterto grasp the state of the hydrocarbon adsorbent is corrected based onthe characteristic changes of the humidity sensors which are detectedbased on the output data of the humidity sensors under a certaincondition (e.g., a condition with respect to a timing to acquire theoutput data of the humidity sensor, an operating state of the internalcombustion engine, or the like). It is thus possible to obtain aparameter where the effect of the characteristic changes of the humiditysensors has been compensated for. Using such a parameter, the state ofthe hydrocarbon adsorbent (a deteriorated state of the hydrocarbonadsorbent, a state in which moisture is adsorbed by the hydrocarbonadsorbent, or the like) can appropriately be grasped while compensatingfor the effect of the characteristic changes of the humidity sensors. Asthe effect of the characteristic changes of the humidity sensors can becompensated for, the requirements for the steadiness of thecharacteristics of the humidity sensors are lessened, and the humiditysensors are allowed to have certain characteristic changes. Therefore,the costs needed to develop and manufacture the humidity sensors can bereduced.

According to the present invention (seventeenth invention), as with thefirst invention, the parameter may be a parameter representative of thestate of the hydrocarbon adsorbent to be grasped, a parameter such as athreshold to be compared with such a parameter, or an intermediateparameter for use in a process for determining such parameters. Thecharacteristic changes of the humidity sensors that are detected by thecharacteristic change detecting means may, for example, becharacteristic changes of the humidity sensors with respect to desiredreference characteristics, such as characteristics of brand-new humiditysensors.

According to the present invention (seventeenth invention), if theapparatus has integrated moisture quantity data generating means forsequentially generating data of an integrated amount of moisturesupplied to the hydrocarbon adsorbent by the exhaust gas emitted fromthe internal combustion engine after the internal combustion engine hasstarted to operate, then the characteristic change detecting meansdetects the characteristic changes of the humidity sensors based on achange due to the characteristic changes of the humidity sensors, ofchanges of transitional characteristics of the output data of thehumidity sensors with respect to the data generated by the integratedmoisture quantity data generating means after the internal combustionengine has started to operate (eighteenth invention).

Alternatively, the characteristic change detecting means may detect thecharacteristic change of the humidity sensor based on a change due tothe characteristic changes of the humidity sensors, of changes oftransitional characteristics of the output data of the humidity sensorswith respect to a period of time that has elapsed after the internalcombustion engine has started to operate (nineteenth invention).

Specifically, as described above with respect to the second or thirdinvention, the output data of the humidity sensors near the hydrocarbonadsorbent exhibits a characteristic transition with respect to the data(the data generated by the integrated moisture quantity data generatingmeans) of the integrated amount of moisture or a period of time that haselapsed after the internal combustion engine has started to operate. Thetransitional characteristics of the output data of the humidity sensorschange depending on the characteristic changes of the humidity sensors.The transitional characteristics relative to the humidity sensorsinclude a portion which changes due to only the characteristic changesof the humidity sensors. Based on a change due to the characteristicchanges of the humidity sensors, of changes of the transitionalcharacteristics relative to the humidity sensors, therefore, thecharacteristic changes of the humidity sensors corresponding to thetransitional characteristics can appropriately be detected. Since thetransitional characteristics relative to the humidity sensors are alsosubject to the effect of the state in which the hydrocarbon adsorbentadsorbs moisture, it is also possible to grasp the state in which thehydrocarbon adsorbent adsorbs moisture while grasping the characteristicchanges of the humidity sensors, based on the transitionalcharacteristics.

The data representative of the integrated amount of moisture in theeighteenth invention may be data substantially proportional to theintegrated amount of moisture (including the integrated amount ofmoisture itself). For example, the integrated value of an amount of fuelor an amount of intake air supplied to the internal combustion engineafter the internal combustion engine has started to operate may be usedas the data representative of the integrated amount of moisture. If theoperating state of the internal combustion engine after the internalcombustion engine has started to operate is a substantially constantoperating state, e.g., an idling state, then the period of time that haselapsed after the internal combustion engine has started to operate maybe used as the data representative of the integrated amount of moisture.

According to the present invention (eighteenth invention) wherein thecharacteristic changes of the humidity sensors are detected based on achange of the transitional characteristics of the output data of thehumidity sensors with respect to the data representative of theintegrated amount of moisture after the internal combustion engine hasstarted to operate, more specifically, the characteristic changedetecting means detects the characteristic changes of the humiditysensors based on a change from a predetermined reference value ofcharacteristic change detecting output data corresponding to therespective humidity sensors which comprise the output data of thehumidity sensors at the time when the data generated by the integratedmoisture quantity data generating means has reached predetermined valuesfor the respective humidity sensors after the internal combustion enginehas started to operate (twentieth invention).

Similarly, according to the present invention (nineteenth invention)wherein the characteristic changes of the humidity sensors are detectedbased on changes of transitional characteristics of the output data ofthe humidity sensors with respect to the period of time that has elapsedafter the internal combustion engine has started to operate (nineteenthinvention), more specifically, the characteristic change detecting meansdetects the characteristic changes of the humidity sensors based on achange from predetermined reference values of characteristic changedetecting output data of the humidity sensors which comprises the outputdata of the humidity sensors at the time when the period of time thathas elapsed after the internal combustion engine has started to operatehas reached predetermined values for the respective humidity sensors(twenty-first invention).

Specifically, as described above with respect to the fourth or fifthinvention, the humidity near the hydrocarbon adsorbent associated withthe humidity sensors changes from a low humidity level to a highhumidity level after the internal combustion engine has started tooperate, until it finally reaches a substantially constant high humiditylevel (the humidity of the exhaust gas when the hydrocarbon adsorbent orthe like does not adsorb moisture). The output data of the humiditysensors at the time when the humidity at the locations of the humiditysensors reaches the humidity inherent in the exhaust gas is of asubstantially constant value. When the humidity sensors suffer acharacteristic change due to their deterioration, a timing at which theoutput data of the humidity sensors becomes substantially constant orthe substantially constant level changes depending on the characteristicchanges.

According to the present invention (twentieth invention or twenty-firstinvention), the output data of the humidity sensors at the time when thedata generated by the integrated moisture quantity data generating means(the data representative of the integrated amount of moisture) hasreached predetermined values for the respective humidity sensors or thetime when the period of time that has elapsed after the internalcombustion engine has started to operate has reached predeterminedvalues for the respective humidity sensors is used as the characteristicchange detecting output data. The predetermined values for therespective humidity sensors may be determined such that the humiditiesat the locations of the humidity sensors corresponding to thepredetermined values reach the inherent humidity of the exhaust gas (asubstantially constant high humidity level) or a humidity close theretowhen the data representative of the integrated amount of moisture or theelapsed period of time has reached the predetermined values. Accordingto the present invention, the characteristic changes of the humiditysensors are detected based on a change from the reference value of thecharacteristic change detecting output data of the respective humiditysensors. Thus, the characteristic changes of the humidity sensors canappropriately be detected.

According to the present invention (eighteenth invention or twentiethinvention) wherein the characteristic changes of the humidity sensorsare detected based on a change of the transitional characteristics ofthe output data of the humidity sensors with respect to the datarepresentative of the integrated amount of moisture after the internalcombustion engine has started to operate, more specifically, if thestate of the hydrocarbon adsorbent to be monitored comprises adeteriorated state of the hydrocarbon adsorbent, then the humiditysensors comprise a downstream humidity sensor disposed downstream of thehydrocarbon adsorbent and an upstream humidity sensor disposed upstreamof the hydrocarbon adsorbent. The apparatus further comprises upstreamchanging timing detecting means for detecting a changing timing at whicha humidity represented by the output data of the upstream humiditysensor changes to a tendency to increase monotonously from a lowhumidity level to a high humidity level after the internal combustionengine has started to operate, and downstream changing timing detectingmeans for detecting a changing timing at which a humidity represented bythe output data of the downstream humidity sensor changes to a tendencyto increase monotonously from a low humidity level to a high humiditylevel after the internal combustion engine has started to operate,wherein the difference between the data generated by the integratedmoisture quantity data generating means at the changing timing detectedby the upstream changing timing detecting means and the data generatedby the integrated moisture quantity data generating means at thechanging timing detected by the downstream changing timing detectingmeans is used as the parameter for grasping the deteriorated state ofthe hydrocarbon adsorbent (twenty-second invention).

Similarly, according to the present invention (nineteenth invention ortwenty-first invention) wherein the characteristic changes of thehumidity sensors are detected based on a change of the transitionalcharacteristics of the output data of the humidity sensors with respectto the period of time that has elapsed after the internal combustionengine has started to operate, if the state of the hydrocarbon adsorbentto be monitored comprises a deteriorated state of the hydrocarbonadsorbent, then the humidity sensors comprise a downstream humiditysensor disposed downstream of the hydrocarbon adsorbent and an upstreamhumidity sensor disposed upstream of the hydrocarbon adsorbent. Theapparatus further comprises upstream changing timing detecting means fordetecting a changing timing at which a humidity represented by theoutput data of the upstream humidity sensor changes to a tendency toincrease monotonously from a low humidity level to a high humidity levelafter the internal combustion engine has started to operate, downstreamchanging timing detecting means for detecting a changing timing at whicha humidity represented by the output data of the downstream humiditysensor changes to a tendency to increase monotonously from a lowhumidity level to a high humidity level after the internal combustionengine has started to operate, and integrated moisture quantity datagenerating means for generating data of an integrated amount of moisturesupplied to the hydrocarbon adsorbent by the exhaust gas emitted fromthe internal combustion engine from the changing timing detected by theupstream changing timing detecting means to the changing timing detectedby the downstream changing timing detecting means, wherein the datagenerated by the integrated moisture quantity data generating means isused as the parameter for grasping the deteriorated state of thehydrocarbon adsorbent (twenty-third invention).

Specifically, the humidity at the location of the downstream humiditysensor disposed downstream of the hydrocarbon adsorbent changes to atendency to increase from a low humidity level to a high humidity levelwhen the adsorption of moisture in the exhaust gas by the hydrocarbonadsorbent is saturated after the internal combustion engine has startedto operate. The changing timing (hereinafter referred to as “downstreamchanging timing”) signifies a timing at which the adsorption of moisturein the exhaust gas by the hydrocarbon adsorbent is saturated (absorptionsaturation timing). The humidity at the location of the upstreamhumidity sensor disposed upstream of the hydrocarbon adsorbent changesto a tendency to increase from a low humidity level to a high humiditylevel when the highly humid exhaust gas generated by the internalcombustion engine after the internal combustion engine has started tooperate reaches a region near the inlet of the hydrocarbon adsorbent.The changing timing (hereinafter referred to as “upstream changingtiming”) signifies a timing at which the hydrocarbon adsorbent starts toessentially adsorb moisture in the exhaust gas (absorption starttiming).

According to the twenty-second invention, therefore, the differencebetween the data generated by the integrated moisture quantity datagenerating means at the downstream changing timing (the datarepresentative of an integrated amount of moisture after the internalcombustion engine has started to operate until the downstream changingtiming) and the data generated by the integrated moisture quantity datagenerating means at the upstream changing timing (the datarepresentative of an integrated amount of moisture after the internalcombustion engine has started to operate until the upstream changingtiming) corresponds to a total amount of moisture that is actuallyadsorbed by the hydrocarbon adsorbent (a maximum amount of moisture thatcan be adsorbed by the hydrocarbon adsorbent) and hence represents adeteriorated state of the hydrocarbon adsorbent. Basically, therefore,the above difference can be used as a parameter (basic parameter) forgrasping the deteriorated state of the hydrocarbon adsorbent.

Similarly, according to the twenty-third invention, the data generatedby the integrated moisture quantity data generating means, i.e., thedata representative of an integrated amount of moisture from theupstream changing timing to the downstream changing timing, correspondsto a total amount of moisture that is actually adsorbed by thehydrocarbon adsorbent (a maximum amount of moisture that can be adsorbedby the hydrocarbon adsorbent) and hence represents a deteriorated stateof the hydrocarbon adsorbent. Consequently, the data generated by theintegrated moisture quantity data generating means can be used as aparameter (basic parameter) for evaluating the deteriorated state of thehydrocarbon adsorbent.

The upstream changing timing and the downstream changing timing aregenerally affected by the effect of the characteristic changes of theupstream humidity sensor and the downstream humidity sensor. However,the effect of the characteristic changes of the humidity sensors can becompensated for by correcting the basic parameter for grasping thedeteriorated state of the hydrocarbon adsorbent depending on thecharacteristic changes of the humidity sensors. Therefore, thedeteriorated state of the hydrocarbon adsorbent can appropriately begrasped using the corrected parameter while compensating for the effectof the characteristic changes of the humidity sensors.

As described in detail later on, the upstream changing timing(adsorption start timing) detected based on the output data of theupstream humidity sensor may suffer variations due to the arrangement ofthe exhaust system of the internal combustion engine and the adsorptionof moisture by a catalytic converter that is disposed upstream of thehydrocarbon adsorbent though the characteristics of the upstreamhumidity sensor remain constant. According to the twenty-secondinvention and the twenty-third invention, however, the upstream changingtiming is detected by the upstream humidity sensor, the parameter forgrasping the deteriorated state of the hydrocarbon adsorbent is obtainedfrom the detected timing as a start point, and the parameter iscorrected depending on the characteristic change of the upstreamhumidity sensor. Therefore, the effect of variations of the upstreamchanging timing can be compensated for.

According to the other aspect of the present invention (seventeenthinvention through twenty-third invention), the characteristic changecompensating means should preferably correct the parameter when eitherone of the characteristic changes of the humidity sensors which aredetected by the characteristic change detecting means exceeds apredetermined quantity (twenty-fourth invention). With this arrangement,when the parameter does not need to be corrected as when characteristicchanges of the humidity sensors are detected due to a temporarydisturbance, the parameter is prevented from being corrected.

In either one of the seventeenth through twenty-fourth inventions, thecharacteristic change compensating means preferably compares thecharacteristic changes of the respective humidity sensors detected bythe characteristic change detecting means with a predetermined upperlimit quantity, and prohibits the deteriorated state of the hydrocarbonadsorbent from being grasped based on the parameter when thecharacteristic change of at least one of the humidity sensors exceedsthe upper limit quantity (twenty-fifth invention). Specifically, ifeither one of the humidity sensors suffers an excessive characteristicchange, the humidity sensor may possibly be excessively deteriorated ormay possibly suffer a failure. In such a situation, it is difficult toobtain the parameter capable of appropriately grasping the state of thehydrocarbon adsorbent. In this case, therefore, the state of thehydrocarbon adsorbent is prohibited from being grasped based on theparameter, and hence is prevented from being grasped in error.

According to the other aspect of the present invention (seventeenththrough twenty-fifth inventions), the humidity sensors preferably haverespective characteristic data holding means for holding, in advance,data of characteristics of individual units of the humidity sensors, andthe characteristic change detecting means preferably detects thecharacteristic changes of the humidity sensors based on the output dataof the humidity sensors and the data held by the characteristic dataholding means (twenty-sixth invention).

With the above arrangement, since the characteristic changes of thehumidity sensors are detected using the data held by the characteristicdata holding means associated with individual units of the humiditysensors, i.e., the data of the characteristics of individual units ofthe humidity sensors, it is possible to compensate for not only theeffect of the characteristic changes of the humidity sensors, but alsothe effect of characteristic variations of individual units of thehumidity sensors in grasping the state of the hydrocarbon adsorbent. Asa result, the requirements for the uniformity of the characteristics ofindividual units of the humidity sensors are lessened, and the costsneeded to develop and manufacture the humidity sensors can be reduced.

If the characteristic changes of the humidity sensors are detected basedon the transitional characteristics of the output data of the humiditysensors with respect to the integrated amount of moisture after theinternal combustion engine has started to operate or if thecharacteristic changes of the humidity sensors are detected based on thetransitional characteristics with respect to the period of time that haselapsed (twentieth invention or twenty-first invention), then thehumidity sensors preferably have characteristic data holding means forholding, in advance, data specifying the reference values relative tothe characteristic change detecting output data of the humidity sensorsas the data of the characteristics of the individual units of thehumidity sensors, and the characteristic change detecting meanspreferably acquires the characteristic change detecting output data ofthe humidity sensors using the reference values of the respectivehumidity sensors which are specified by the data held by thecharacteristic data holding means (twenty-seventh invention).

With the above arrangement, the reference value serving as a referencefor detecting the characteristic changes of the humidity sensors isadjusted so as to match the characteristics of the individual units ofthe humidity sensors. As a consequence, the characteristic changes ofthe humidity sensors can be detected while appropriately compensatingfor the effect of characteristic variations of the individual units ofthe humidity sensors.

According to the present invention (twenty-sixth or twenty-seventhinvention) wherein the effect of characteristic variations of individualunits of the humidity sensors is compensated for, the characteristicdata holding means preferably comprises resistive elements havingresistances depending on the value of the data of the characteristics ofthe individual units of the humidity sensors (twenty-eighth invention).

With the above arrangement, the characteristic data holding means can beof an inexpensive and simple structure. Since the resistances of theresistive elements can be detected relatively easily, the data of thecharacteristics of the individual units of the humidity sensors caneasily be recognized. The characteristic data holding means shouldpreferably be associated with connectors by which the humidity sensorsare connected to an electronic circuit unit or the like which processesthe output data of the humidity sensors, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an overall system arrangement of anapparatus according to first and second embodiments of the presentinvention;

FIG. 2 is a diagram showing output characteristics of a humidity sensorused in the apparatus shown in FIG. 1;

FIG. 3 is a block diagram showing an arrangement of a portion of theapparatus shown in FIG. 1;

FIG. 4 is a flowchart of an operation sequence of the apparatusaccording to the first embodiment of the present invention;

FIG. 5 is a graph illustrative of a processing sequence of the flowchartof FIG. 4;

FIG. 6 is a flowchart of an operation sequence of the apparatusaccording to the first embodiment;

FIGS. 7 and 8 are graphs illustrative of a processing sequence of theflowchart of FIG. 6;

FIGS. 9 through 12 are flowcharts of an operation sequence of theapparatus according to the second embodiment of the present invention;

FIGS. 13 and 14 are graphs illustrative of a processing sequence of theflowchart of FIG. 12;

FIG. 15 is a block diagram of an overall system arrangement of anapparatus according to a third embodiment of the present invention;

FIGS. 16 through 18 are flowcharts of an operation sequence of theapparatus according to the third embodiment of the present invention;

FIGS. 19 and 20 are graphs illustrative of a processing sequence of theflowchart of FIGS. 17 and 18; and

FIG. 21 is a view of another exhaust gas purifier having a hydrocarbonadsorbent.

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of the present invention will be described below withreference to FIGS. 1 through 8. FIG. 1 is a block diagram of an overallsystem arrangement of an apparatus according to the present embodiment.In FIG. 1, an engine (an internal combustion engine) 1 mounted on anautomobile or a hybrid vehicle, for example, draws a mixture of fuel andair from an intake pipe 4 having a throttle valve 2 and a fuel injector3 as in a usual engine, and combusts the mixture and generates anexhaust gas, which is discharged into an exhaust pipe (exhaust passage)5. A catalytic converter 6 and an exhaust gas purifier 8 whichincorporates a hydrocarbon adsorbent 7 (hereinafter referred to as “HCadsorbent 7”) are successively arranged downstream and mounted on theexhaust pipe 5 for purifying the exhaust gas emitted from the engine 1.A portion of the exhaust pipe 5 which extends upstream of the catalyticconverter 6 and a portion of the exhaust pipe 5 which extends downstreamof the catalytic converter 6 are referred to as an upstream exhaust pipe5 a and a downstream exhaust pipe 5 b, respectively. The downstreamexhaust pipe 5 b has a downstream end that is open into the atmosphere.If necessary, a catalytic converter separate from the catalyticconverter 6 and a muffler (silencer) or the like may be connected to thedownstream exhaust pipe 5 b downstream of the exhaust gas purifier 8.

The catalytic converter 6 incorporates a three-way catalyst (not shown)therein. The catalytic converter 6 purifies, by way of oxidizing andreducing reactions, gas components including nitrogen oxide (NOx),hydrocarbons (HC), carbon monoxide (CO), etc. contained in the exhaustgas emitted from the engine 1 and supplied from the upstream exhaustpipe 5 a into the catalytic converter 6.

The exhaust gas purifier 8 has a substantially cylindrical housing 9mounted on the downstream exhaust pipe 5 b in covering relation to theouter circumferential surface thereof. The downstream exhaust pipe 5 bextends centrally through the housing 9. A tubular space 10 definedbetween the inner circumferential surface of the housing 9 and the outercircumferential surface of the downstream exhaust pipe 5 b serves as abypass exhaust passage 10 for branching the exhaust gas from thedownstream exhaust pipe 5 b. The HC adsorbent 7 is mounted in the bypassexhaust passage 10. The HC adsorbent 12 is made of a zeolite-basedmaterial and serves to adsorb HC contained in the exhaust gas which isemitted from the engine 1 in an initial phase of operation of theinternal combustion engine 1.

The bypass exhaust passage 10 of the exhaust gas purifier 8 which hasthe HC adsorbent 7 communicates with the interior of the downstreamexhaust pipe 5 b through a plurality of vent holes 11 that are definedin the downstream exhaust pipe 5 b within the housing 9 upstream of theHC adsorbent 7. The bypass exhaust passage 10 also communicates with andis joined to the downstream exhaust pipe 5 b through a joint pipe 12that extends from the housing 9 downstream of the HC adsorbent 7.Furthermore, the bypass exhaust passage 10 is connected to the intakepipe 4 downstream of the throttle valve 2 by an EGR (Exhaust GasRecirculation) passage 13 that extends from the housing 9 downstream ofthe HC adsorbent 7.

The EGR passage 13 serves to return the exhaust gas to the intake pipe 4under given conditions during operation of the engine 1 in order tocombust an unburned gas in the exhaust gas. An on/off valve(solenoid-operated valve) 14 is mounted in the EGR passage 13 forselectively opening and closing the EGR passage 13.

The downstream exhaust pipe 5 b and the joint pipe 12 are joined to eachother at a junction A where there is disposed a directional controlvalve 15 for venting one, at a time, of the portion of the downstreamexhaust pipe 5 b which extends upstream of the junction A and the bypassexhaust passage 10 to the atmosphere. The directional control valve 15can be angularly moved between a solid-line position and animaginary-line position in FIG. 1 by an actuator (not shown). When thedirectional control valve 15 is actuated into the solid-line position,it shields the portion of the downstream exhaust pipe 5 b which extendsupstream of the junction A from the atmosphere, and simultaneously ventsthe bypass exhaust passage 10 to the atmosphere. Conversely, when thedirectional control valve 15 is actuated into the imaginary-lineposition, it vents the downstream exhaust pipe 5 b to the atmosphere andshields the bypass exhaust passage 10 to the atmosphere.

The apparatus also has, in addition to the above mechanical structures,the following components for controlling operation of the engine 1 andmonitoring a state of the HC adsorbent 7. Specifically, the apparatusaccording to the present embodiment has a controller 16 (hereinafterreferred to as “ECU 16”) for controlling operation of the engine 1(including operation of the on/off valve 14 in the EGR passage 13 andthe directional control valve 15), a deterioration evaluating device 17for executing a processing sequence to evaluate a deteriorated state ofthe HC adsorbent 7 as a state of the HC adsorbent 7, a deteriorationindicator 18 for indicating the deteriorated state as evaluated, and ahumidity sensor 19 mounted on the housing 9 downstream of the HCadsorbent 7 for detecting the humidity of the exhaust gas downstream ofthe HC adsorbent 7. The ECU 16 and the deterioration evaluating device17 are implemented by a microcomputer or the like. The deteriorationindicator 18 comprises a lamp, a buzzer, or a display unit fordisplaying characters, a graphic image, etc. Those parts that areindicated by the reference numerals 23, 24 in FIG. 1 are sensors used ina second embodiment to be described later on, and may be omitted in thepresent embodiment.

The humidity sensor 19 has a humidity sensing element (not shown) madeof a porous material of alumina, titania, or the like, whose electricresistance varies depending on the humidity (relative humidity) of theexhaust gas to which the humidity sensor 19 is exposed. When thehumidity sensor 19 is energized by the deterioration evaluating device17, the humidity sensor 19 produces an output voltage VRHUM, which isproportional to the electric resistance of the humidity sensing element,depending on the humidity (relative humidity) of the exhaust gas, asshown in FIG. 2. The output voltage VRHUM of the humidity sensor 19decreases substantially linearly as the humidity increases. Therefore,the output voltage VRHUM of the humidity sensor 19 has a negativehumidity coefficient, i.e., it decreases as the humidity (relativehumidity) of the exhaust gas increases. The reference characters VFHUMin parentheses on the vertical axis in FIG. 2 refer to an output voltageused in a third embodiment of the present invention which will bedescribed later on.

As shown in FIG. 3, the humidity sensor 19 is connected to thedeterioration evaluating device 17 by two connectors 20 a, 20 b. Theconnector 20 a closer to the humidity sensor 19 has a resistive element21 serving as a characteristic data holding means. The resistive element21 has a resistance depending on the characteristics actually measuredfor each individual unit of the humidity sensor 19. The resistiveelement 21 (hereinafter referred to as “label resistive element 21”) iselectrically connected, together with the humidity sensor 19, to thedeterioration evaluating device 17 when the connector 20 a closer to thehumidity sensor 19 is connected to the connector 20 b closer to thedeterioration evaluating device 17. The deterioration evaluating device17 has a resistance detecting circuit 22 for detecting the resistance ofthe label resistive element 21, i.e., generating a voltage having alevel depending on the resistance of the label resistive element 21. Thedeterioration evaluating device 17 recognizes the characteristics of thehumidity sensor 19 that is used based on the resistance of the labelresistive element 21 which is detected by the resistance detectingcircuit 22, and sets a parameter, to be described in detail later on,related to the evaluation of a deteriorated state of the HC adsorbent 7.The reference numerals 25 in parentheses shown in FIG. 3 refer to ahumidity sensor in the third embodiment of the present invention whichwill be described later on.

The ECU 16 is supplied with detected data of the rotational speed NE ofthe engine 1, the engine temperature TW thereof (specifically, thetemperature of the coolant of the engine 1), etc. from non-illustratedsensors, and also with an operation start command signal and anoperation stop command signal for the engine 1 from a non-illustratedoperation switch. The ECU 16 then controls operation of the engine 1based on the detected data and the command signals that are suppliedthereto, according to a predetermined control program. Specifically, theECU 16 controls the opening of the throttle valve 2 with an actuator(not shown), controls the amount of fuel injected by the fuel injector3, controls an igniter (not shown), controls the starting of the engine1 with a starter motor (not shown), controls the on/off valve 14 mountedin the EGR passage 13, and controls operation of the directional controlvalve 15.

The deterioration evaluating device 17 is supplied with the outputvoltage (a signal indicative of a detected relative humidity) from thehumidity sensor 19, detected data of the resistance of the labelresistive element 21 which represents the characteristics of thehumidity sensor 19, and data of the engine temperature TW of the engine1 from the ECU 16. The deterioration evaluating device 21 evaluates(grasps) a deteriorated state of the HC adsorbent 7 of the exhaust gaspurifier 8 based on the supplied data according to a predeterminedprogram, as described later on. The deterioration evaluating device 17evaluates a deteriorated state of the HC adsorbent 7 as either a statewhere it has been deteriorated to the extent that it needs to bereplaced (such a deteriorated state will hereinafter be referred to as“deterioration-in-progress state”) or a state where it has not beendeteriorated to the deterioration-in-progress state (such a state willhereinafter be referred to as “non-deteriorated state”). When thedeterioration evaluating device 17 judges that the deteriorated state ofthe HC adsorbent 12 is the deterioration-in-progress state, thedeterioration evaluating device 17 controls the deterioration indicator18 to indicate the deteriorated state as thus evaluated.

The deterioration evaluating device 17 is capable of exchanging variousdata with the ECU 16, and is capable of giving the ECU 16 data relatedto a process of evaluating a deteriorated state of the HC adsorbent 7,e.g., data indicative of whether the adsorption of moisture by the HCadsorbent 7 has been saturated or not. The deterioration evaluatingdevice 17 functions as a characteristic change detecting means, acharacteristic change compensating means, and an integrated moisturequantity data generating means.

Operation of the apparatus according to the present embodiment,particularly for evaluating the deteriorated state of the HC adsorbent7, will be described in detail below.

When the operation switch (not shown) is turned on to start operation ofthe engine 1, the ECU 16 and the deterioration evaluating device 17 areactivated. The ECU 16 operates the directional control valve 15 to moveto the solid-line position in FIG. 1 with the non-illustrated motor. Thedownstream exhaust pipe 5 b is shielded at the junction A, and thebypass exhaust passage 10 of the exhaust gas purifier 8 is vented to theatmosphere. The ECU 16 then starts the engine 1 with the starter motor(not shown) to rotate the crankshaft (not shown) of the engine 1. TheECU 16 controls the fuel injector 3 to supply the fuel to the engine 1,and controls the igniter (not shown) to start operating the engine 1.

When the engine 1 starts operating, it emits an exhaust gas that isdischarged through the upstream exhaust pipe 5 a, the catalyticconverter 6, the upstream portion of the downstream exhaust pipe 5 b(extending from the catalytic converter 6 to the vent holes 11), thebypass exhaust passage 10, the HC adsorbent 7, the joint pipe 12, andthe downstream portion of the downstream exhaust pipe 5 b (extendingdownstream of the directional control valve 15) into the atmosphere. Atthis time, while the exhaust gas is passing through the HC adsorbent 7in the bypass exhaust passage 10, hydrocarbons (HCs) in the exhaust gasare adsorbed by the HC adsorbent 7. Therefore, even while the catalyticconverter 6 is not sufficiently activated as when the engine 1 starts tooperate at a low temperature, the HCs are prevented from beingdischarged into the atmosphere. At the same time, the HC adsorbent 7adsorbs moisture in the exhaust gas as well as the HCs in the exhaustgas.

When the catalytic converter 6 is sufficiently activated by being heatedby the exhaust gas, the directional control valve 15 is moved to theimaginary-line position in FIG. 1, venting the downstream exhaust pipe 5b to the atmosphere and shielding the bypass exhaust passage 10 of theexhaust gas purifier 8 from the atmosphere. The ECU 16 opens the on/offvalve 14 in the EGR passage 13 under predetermined operating conditionsof the engine 1. At this time, HCs that are released from the HCadsorbent 7 after the HC adsorbent 7 is heated by the exhaust gas flowthrough the EGR passage 13 into the intake pipe 4, and then combusted inthe engine 1.

After the deterioration evaluating device 17 is activated, it performs aprocessing sequence shown in FIG. 4. The process shown in FIG. 4 isperformed only when the engine 1 starts operating.

The deterioration evaluating device 17 determines the value of a flagF/HCPG in STEP1. The flag F/HCPG is “1” if the release of HCs adsorbedby the HC adsorbent 7 in a preceding operation of the engine 1 iscompleted, and “0” if not completed. The flag F/HCPG is set by the ECU16 while the engine 1 is in operation. When the temperature of the HCadsorbent 7 is equal to or higher than a temperature at which HCsadsorbed by the HC adsorbent 7 are released, the ECU 16 determines anintegrated value of the amount of gas that has flowed through the EGRpassage 13 when the on/off valve 14 in the EGR passage 13 is open.

If the determined integrated value becomes equal to or higher than apredetermined value, then the ECU 16 judges that the release of HCsadsorbed by the HC adsorbent 7 is completed (at this time, the releaseof moisture adsorbed by the HC adsorbent 7 is also completed). The ECU16 thus sets the flag F/HCPG to “1”. If the determined integrated valueis lower than the predetermined value, then the ECU 16 sets the flagF/HCPG to “0”. The flag F/HCPG is stored in a nonvolatile memory such asan EEPROM or the like (not shown) when the engine 1 is shut off, so thatthe flag F/HCPG will not be lost when the engine 1 is not operating.

If F/HCPG=0 in STEP1, then since the release of HC and moisture from theHC adsorbent 12 is not completed in the preceding operation of theengine 1, i.e., the HC adsorbent 7 has already adsorbed moisture in thepresent operation of the engine 1, the deterioration evaluating device17 sets a flag F/MCND to “0” in STEP10, and puts the operation sequenceshown in FIG. 4 to an end. If the flag F/MCND is set to “0”, then itmeans that the apparatus is in a state not suitable for evaluating thedeteriorated state of the HC adsorbent 7 or the present process ofevaluating the deteriorated state of the HC adsorbent 7 has already beenfinished. If the flag F/MCND is set to “1”, then it means that theapparatus is in a state to evaluate a deteriorated state of the HCadsorbent 7.

If F/HCPG=1 in STEP1, then the deterioration evaluating device 17acquires from the ECU 16 detected data representative of the presentengine temperature TW (hereinafter referred to as “initial enginetemperature TW”) of the engine 1 as data representative of thetemperature of the HC adsorbent 7 at the start of operation of theengine 1 in STEP2. If the apparatus has a temperature sensor fordetecting the temperature of the HC adsorbent 7 or a nearby region, thenthe above data may be detected from the temperature sensor.

Thereafter, the deterioration evaluating device 17 determines whetherthe engine temperature TW is in a predetermined range (TWL≦TW≦THW) ornot in STEP3. If the engine temperature TW is not in the predeterminedrange, then the deterioration evaluating device 17 judges that theapparatus is not in a state capable of adequately evaluating thedeteriorated state of the HC adsorbent 7, and sets the flag F/MCND to“0” in STEP4 and then puts the operation sequence shown in FIG. 4 to anend. This is because the deteriorated state of the HC adsorbent 7 cannotadequately be evaluated if the temperature of the HC adsorbent 7 isexcessively low (e.g., 0° C. or lower) or if the temperature of the HCadsorbent 7 is excessively high (e.g., 50° C. or higher).

In addition to determining the conditions in STEP1 and STEP3, thedeterioration evaluating device 17 may determine whether soaking priorto the start of the present operation of the engine 1 has been completedor not. Specifically, the deterioration evaluating device 17 maydetermine whether the temperature of the engine 1 and its exhaust system(the exhaust gas purifier 8, etc.) has dropped to a temperature (steadytemperature) which is about the same as the atmospheric temperatureafter the preceding operation of the engine 1 has stopped. Thecompletion of soaking may be determined based on whether the period oftime that has elapsed after the engine 1 has stopped operating is apredetermined period of time (e.g., four hours) or more, or whether theengine temperature TW of the engine 1 has substantially converged to theatmospheric temperature. If it is judged that soaking is not completed,then the deterioration evaluating device 17 may judge that the apparatusis not in a state capable of adequately evaluating the deterioratedstate of the HC adsorbent 7, and may set the flag F/MCND to “0”. This isbecause when soaking is not completed, the humidity (relative humidity)in the vicinity of the HC adsorbent 7 and the ability of the HCadsorbent 7 to adsorb moisture tend to be unstable due to the remainingheat of the engine 1 and the HC adsorbent 7.

In the present embodiment, if the initial engine temperature TW is inthe above predetermined range in STEP3, then the deteriorationevaluating device 17 sets the value of the flag F/MCND to “1” in orderto carry out the process of evaluating the deteriorated state of the HCadsorbent 7 in STEP4. If it is determined whether soaking has beencompleted or not, then the deterioration evaluating device 17 sets thevalue of the flag F/MCND to “1” if soaking has been completed and, inaddition, the conditions of STEP1, STEP3 have been satisfied.

Thereafter, the deterioration evaluating device 17 acquires present dataof the output voltage VRHUM of the humidity sensor 19 (detected data ofthe relative humidity) from the humidity sensor 19, and also acquiresdata of the resistance LBLR of the label resistive element 21 throughthe resistance detecting circuit 22 in STEP5.

The deterioration evaluating device 17 then stores the present value ofthe output voltage VRHUM of the humidity sensor 19 acquired in STEP5 asthe initial value of a parameter VRHUM/MAX (hereinafter referred to as“maximum output parameter VRHUM/MAX”) representative of the latest valueof a maximum value of the output voltage VRHUM of the humidity sensor 19and the initial value of a parameter VRHUM/PRE (hereinafter referred toas “preceding output parameter VRHUM/PRE”) representative of a precedingvalue of the output voltage VRHUM (a preceding value in each cycle timeof the processing sequence shown in FIG. 6 of the deteriorationevaluating device 17) in STEP6. Since the output voltage VRHUM of thehumidity sensor 19 according to the present embodiment has a negativehumidity coefficient, i.e., it decreases as the relative humidityincreases, as described above, the maximum value of the output voltageVRHUM of the humidity sensor 19 corresponds to the minimum value of therelative humidity (the relative humidity grasped from the output voltageVRHUM based on the characteristics shown in FIG. 2) that is detected bythe humidity sensor 19.

Then, the deterioration evaluating device 17 determines a deteriorationevaluating threshold TRSTMDT with which to determine whether the HCadsorbent 7 is in the deterioration-in-progress state or thenon-deteriorated state from the detected data of the initial enginetemperature TW acquired in STEP2 according to a predetermined data tableas indicated by the solid-line curve in FIG. 5 in STEP7. Thedeterioration evaluating threshold TRSTMDT corresponds to the maximumamount of moisture that can be adsorbed by the HC adsorbent 7. As theinitial humidity of the HC adsorbent 7 is lower, the maximum amount ofmoisture that can be adsorbed by the HC adsorbent 7, and hence themaximum amount of HCs that can be adsorbed by the HC adsorbent 7, aregreater. The data table as indicated by the solid-line curve in FIG. 5is determined by way of experimentation or the like such that thedeterioration evaluating threshold TRSTMDT is greater as the initialengine temperature TW of the engine 1 is lower. According to the presentembodiment, the period of time that has elapsed after the start of theoperation of the engine 1 is used to grasp the total amount of moisturethat has been adsorbed by the HC adsorbent 7 after the start of theoperation of the engine 1. Therefore, the deterioration evaluatingthreshold TRSTMDT is a threshold relative to the period of time that haselapsed after the start of the operation of the engine 1. A graph asindicated by the broken-line curve in FIG. 5 is related to the thirdembodiment which will be described later on.

Then, the deterioration evaluating device 17 sets the values ofparameters TM/SH, TMV/TSH, and VRHUM/INI for compensating for the effectof variations of the characteristics of individual units of the humiditysensor 19 in the process, to described later on, for evaluating thedeteriorated state of the HC adsorbent 7, from the detected data of theresistance LBLR of the label resistive element 21 which has beenacquired in STEP5, in STEP8. These parameters TM/SH, TMV/TSH, andVRHUM/INI, whose meanings will be described later on, are determinedfrom the detected data of the resistance LBLR of the label resistiveelement 21 based on predetermined data tables corresponding respectivelythereto.

Then, the deterioration evaluating device 17 initializes, to “0”, thevalue of a timer TM (count-up timer) which measures a period of timethat has elapsed from the start of the operation of the engine 1, andalso initializes a flag F/RST, to be described later on, to “0” inSTEP9. Thereafter, the processing sequence shown in FIG. 4 is ended.

After having carried out the processing sequence shown in FIG. 4 whenthe engine 1 starts to operate, the deterioration evaluating device 17carries out a processing sequence shown in FIG. 6 in a given cycle timeto evaluate the deteriorated state of the HC adsorbent 7. According tothe present embodiment, the processing sequence shown in FIG. 6 isperformed while the engine 1 is idling immediately after the engine 1has started to operate.

Prior to specifically describing the processing sequence shown in FIG.6, a basic concept of the time-dependent transition of the outputvoltage VRHUM of the humidity sensor 19 after the engine 1 has startedto operate and a process of evaluating the deteriorated state of the HCadsorbent 7 according to the present embodiment will first be describedbelow.

When the engine 1 starts operating, it emits an exhaust gas that issupplied through the exhaust system downstream of the engine 1 to the HCadsorbent 7. At this time, since the exhaust system downstream of theengine 1 and the HC adsorbent 7 have their temperatures equal to orlower than the dew point of moisture in the exhaust gas, the relativehumidity of the exhaust gas upstream of the HC adsorbent 7 is asubstantially constant relatively high humidity (about 100%). When theexhaust gas is supplied to the HC adsorbent 7, moisture as well as HCsin the exhaust gas are adsorbed by the HC adsorbent 7. Therefore, therelative humidity of the exhaust gas at the location of the humiditysensor 19 downstream of the HC adsorbent 7 is relatively low, and theoutput voltage VRHUM of the humidity sensor 19 is a voltage having arelatively high level.

At this time, though the output voltage VRHUM of the humidity sensor 19downstream of the HC adsorbent 7 slightly varies due to the effect ofdisturbances, it has a generally high constant level as indicated by thesolid-line curve a, for example, in FIG. 7 immediately after the engine1 has started to operate (the relative humidity of the exhaust gas atthe location of the humidity sensor 19 is of a generally constant lowlevel).

As the adsorption of moisture by the HC adsorbent 7 progresses until itbecomes saturated (the adsorption of HCs by the HC adsorbent 7 alsobecomes saturated), the HC adsorbent 7 no longer adsorbs moisture. Thus,the relative humidity downstream of the HC adsorbent 7 increasesmonotonously toward a high relative humidity level inherent in theexhaust gas, i.e., the relative humidity of the exhaust gas upstream ofthe HC adsorbent 7. Thus, the relative humidity changes to a tendency toincrease monotonously from a low humidity level toward a high humiditylevel. Therefore, the output voltage VRHUM of the humidity sensor 19changes to a tendency to decrease monotonously from a high voltage leveltoward a low voltage level which corresponds to the relative humidity(substantially constant) inherent in the exhaust gas.

The integrated amount of moisture that is supplied to the HC adsorbent 7after the engine 1 has started to operate up to the timing (changingtiming) at which the output voltage VRHUM of the humidity sensor 16changes from a high voltage level to the tendency to decreasemonotonously, i.e., until the adsorption of moisture by the HC adsorbent7, depends on the period of time that has elapsed from the start ofoperation of the engine 1 (hereinafter referred to as “engine operationelapsed time”) while the engine 1 is idling, for example. As the HCadsorbent 7 is more deteriorated, the amounts of moisture and HCs thatcan be adsorbed by the HC adsorbent 7 are reduced. Therefore, the timing(changing timing) at which the output voltage VRHUM of the humiditysensor 16 changes to the tendency to decrease monotonously after theengine 1 has started to operate becomes earlier as the HC adsorbent 7 ismore deteriorated.

According to the present embodiment, basically, the changing timing(time t1 in FIG. 7) at which the output voltage VRHUM of the humiditysensor 19 changes from the high voltage level to the tendency todecrease monotonously after the engine 1 has started to operate isdetected, and an engine operation elapsed time TMTRS/PM at that changingtiming is obtained as a basic parameter representative of thedeteriorated state of the HC adsorbent 7. The basic parameter is thencompared with a predetermined threshold to evaluate the deterioratedstate of the HC adsorbent 7. In the present embodiment, the engineoperation elapsed time serves as data representative of the integratedamount of moisture that is supplied by the exhaust gas to the HCadsorbent 7 after the engine 1 has started to operate.

The above transition of the output voltage VRHUM of the humidity sensor19 is also affected by characteristic changes of the humidity sensor 19due to aging thereof. The solid-line curve a in FIG. 7 represents thecharacteristics of the humidity sensor 19 when it is brand-new. When thehumidity sensor 19 suffers characteristic changes due to deteriorationthereof, the output voltage VRHUM of the humidity sensor 19 exhibitstransitional characteristics as indicated by the broken-line curve b,for example, in FIG. 7 after the engine 1 has started to operate.Specifically, when the humidity sensor 19 suffers characteristic changesdue to deterioration thereof, the changing timing (time t2 in FIG. 7) atwhich the output voltage VRHUM of the humidity sensor 19 changes to thetendency to decrease monotonously due to the saturation of theadsorption of moisture by the HC adsorbent 7 is later than if thehumidity sensor 19 is normal (if the humidity sensor 19 is brand-new).Even when the relative humidity of the exhaust gas at the location ofthe humidity sensor 19 finally becomes its inherent relative humidity(substantially 100%) due to the saturation of the HC adsorbent 7, theoutput voltage VRHUM of the humidity sensor 19 is of a level higher thanif the humidity sensor 19 is normal. Thus, the output voltage VRHUM ofthe humidity sensor 19 undergoes an offset. Though the changing timingat which the output voltage VRHUM of the humidity sensor 19 changes tothe tendency to decrease monotonously is affected by the deterioratedstate of the humidity sensor 19, the output voltage VRHUM of thehumidity sensor 19 at the time the relative humidity of the exhaust gasat the location of the humidity sensor 19 becomes its inherent relativehumidity (substantially 100%) basically changes only due to thecharacteristic changes of the humidity sensor 19.

The transitional characteristics of the output voltage VRHUM of thehumidity sensor 19 also suffer slight variations due to variations ofthe response characteristics of different individual units of thehumidity sensor 19 even if they are deteriorated to the same extent. Forexample, the period of time (time TMVR/TSH in FIG. 7) required for theoutput voltage VRHUM of the humidity sensor 19 to reach a low levelvoltage or a voltage close thereto which corresponds to the inherentrelative humidity of the exhaust gas after the HC adsorbent 7 issaturated, suffers slight variations among different individual units ofthe humidity sensor 19 due to variations of the response characteristicsof those different individual units of the humidity sensor 19 even ifthey are brand-new. Furthermore, the output voltage VRHUM (VRHUM/INI inFIG. 7) of the humidity sensor 19 when the relative humidity of theexhaust gas at the location of the humidity sensor 19 is a substantiallyconstant inherent relative humidity also suffers slight variations amongdifferent individual units of the humidity sensor 19 due to variationsof the circuit characteristics of those different individual units ofthe humidity sensor 19 even if they are brand-new.

In the process of evaluating the deteriorated state of the HC adsorbent7 according to the present embodiment, the above characteristic changesof the humidity sensor 19 and variations of the characteristics ofdifferent individual units of the humidity sensor 19 are compensatedfor.

Based on the concept described above, the processing sequence accordingto the flowchart shown in FIG. 6 will be described below. Thedeterioration evaluating device 17 carries out an operation sequenceshown in FIG. 6 in a given cycle time after the engine 1 has beenactivated. According to the operation sequence shown in FIG. 6, thedeterioration evaluating device 17 determines the value of the flagF/MCND set in the processing sequence shown in FIG. 4 in STEP11. IfF/MCND=0, then it means that the apparatus is in a state not suitablefor evaluating the deteriorated state of the HC adsorbent 7 or thepresent process of evaluating the deteriorated state of the HC adsorbent7 has already been finished. Therefore, the deterioration evaluatingdevice 17 puts the processing sequence shown in FIG. 6 to an end.

If F/MCND=1, then the deterioration evaluating device 17 increments thevalue of the timer TM, which has been initialized to “0” in theprocessing sequence shown in FIG. 4 when the engine 1 starts to operate,for measuring the engine operation elapsed time, by a predeterminedvalue ΔTM (fixed value) in STEP12, and then determines the value of aflag F/RST in STEP13. The flag F/RST is “1” when the detection of thechanging timing at which the output voltage VRHUM of the humidity sensor19 changes from the high level voltage to the tendency to decreasemonotonously is finished, and “0” when the detection of the changingtiming is not finished. Inasmuch as the value of the flag F/RST isinitialized when the engine 1 starts to operate, F/RST=0 immediatelyafter the engine 1 has started to operate. Because the value of thetimer TM represents the engine operation elapsed time, the engineoperation elapsed time will hereinafter be denoted by TM.

If F/RST=0 in STEP13, then the deterioration evaluating device 17acquires the present data of the output voltage VRHUM of the humiditysensor 19 in STEP14, and compares the value of the relative humidityVHUMD and the preceding output parameter VRHUM/PRE with each other inSTEP15. If VRHUM>VRHUM/PRE, then the deterioration evaluating device 17updates the value of the maximum output parameter VRHUM/MAX with thepresent value of the output voltage VRHUM of the humidity sensor 19 inSTEP16, and thereafter updates the value of the preceding outputparameter VRHUM/PRE with the present value of the output voltage VRHUMin STEP17. If VRHUM≦VRHUM/PRE in STEP15, then the deteriorationevaluating device 17 does not update the value of the maximum outputparameter VRHUM/MAX, but updates the value of the preceding outputparameter VRHUM/PRE in STEP17.

According to the processing in STEP15 through STEP17, after the engine 1has started to operate, the maximum value of the output voltage VRHUM ofthe humidity sensor 19 (the minimum value of the relative humidity whichis represented by the output voltage VRHUM) is sequentially detected.

Then, the deterioration evaluating device 17 compares the present valueof the output voltage VRHUM of the humidity sensor 19 with the value(VRHUM/MAX−VRHUM/JUD) which is produced by subtracting a predeterminedvalue VRHUM/JUD from the present value of the maximum output parameterVRHUM/MAX in STEP18. If VRHUM≧VRHUM/MAX−VRHUM/JUD, then it is judgedthat the timing of the present cycle time is not the changing timing atwhich the output voltage VRHUM of the humidity sensor 19 changes to thetendency to decrease monotonously (the timing at which the adsorption ofmoisture and HCs by the HC adsorbent 7 is saturated, hereinafter alsoreferred to as “adsorption saturation timing”), and the presentprocessing sequence shown in FIG. 6 is ended.

If VRHUM<VRHUM/MAX−VRHUM/JUD in STEP18, then the deteriorationevaluating device 17 judges that the timing of the present cycle time isthe adsorption saturation timing (the time t1 or t2 in FIG. 7), andcompares the present engine operation elapsed time TM with the value ofthe parameter TM/SH (see FIG. 7) that is set depending on the resistanceLBLR of the label resistive element 21 according to the processingsequence shown in FIG. 4 in STEP19. The parameter TM/SH is signified asan upper limit for the engine operation elapsed time TM at a timing thatis appropriate as the adsorption saturation timing. Since the parameterTM/SH is set depending on the resistance of the label resistive element21, it matches the individual characteristics of the humidity sensor 19.

If TM<TM/SH in STEP19, then the deterioration evaluating device 17judges that the timing of the present cycle time is an appropriateadsorption saturation timing, and stores the present engine operationelapsed time TM as the value (basic value) of a deterioration evaluatingparameter TMTRS/PM for evaluating the deteriorated state of the HCadsorbent 7 in STEP20. Since the detection of the adsorption saturationtiming is properly finished in this case, the deterioration evaluatingdevice 17 sets the value of the flag F/RST to “1” in STEP21, and putsthe present processing sequence shown in FIG. 6 to an end. In this case,therefore, F/RST=1 in STEP13 from the next cycle time, and theprocessing from STEP23, to be described later on, is performed.

If TM≧TM/SH in STEP19, then since the adsorption saturation timing isexcessively later than and inappropriate for the individualcharacteristics of the humidity sensor 19, the deterioration evaluatingdevice 17 sets the value of the flag F/MCND to “0” in STEP22, and putsthe present processing sequence shown in FIG. 6 to an end. In this case,therefore, F/MCND=0 in STEP11 from the next cycle time, and the presentprocessing sequence shown in FIG. 6 is immediately put to an end. Thestate in which TM≧TM/SH in STEP19 is basically a state in which it ishighly possible for the humidity sensor 19 to be suffering a failure,and usually TM<TM/SH in this state. Therefore, the parameter TM/SH isnot necessarily required to be set for each individual unit of thehumidity sensor 19, but may be set to a predetermined fixed value inview of variations of individual units of the humidity sensor 19.

According to the processing sequence described above, the time (the timet1 with respect to the curve a or the time t2 with respect to the curveb in FIG. 7) at which the output voltage VRHUM of the humidity sensor 19has decreased the predetermined value VRHUM/JUD from the finallyacquired maximum value VRHUM/MAX after the engine 1 has started tooperate is detected as the adsorption saturation timing (the changingtiming at which the output voltage VRHUM changes to the tendency todecrease monotonously), and the engine operation elapsed time TM (whichcorresponds to the integrated amount of moisture supplied to the HCadsorbent until the adsorption saturation timing) at the adsorptionsaturation timing is obtained as the basic value of the deteriorationevaluating parameter TMTRS/PM. If the parameter TM/SH (see FIG. 7) isprovided which determines the upper limit for the engine operationelapsed time TM at the adsorption saturation timing, and the parameterTM/SH is set depending on the resistance of the label resistive element21 corresponding to the characteristics of individual units of thehumidity sensor 19, an inappropriate deterioration evaluating parameterTMTRS/PM is prevented from being obtained while compensating forcharacteristic variations among individual units of the humidity sensor19.

After the deterioration evaluating parameter TMTRS/PM has thus beenobtained, since F/RST=1 in STEP13, the deterioration evaluating device17 compares the present engine operation elapsed time TM with theparameter TMVR/TSH which has been set depending on the resistance of thelabel resistive element 21 in the processing sequence shown in FIG. 4(depending on the characteristics of the individual unit of the humiditysensor 19) in STEP23. Referring to FIG. 7, the parameter TMVR/TSH issignified as a reference value of the engine operation elapsed time TMuntil the output voltage VRHUM of the humidity sensor 19 reaches a value(a substantially constant value) corresponding to the inherent humidityof the exhaust gas after the adsorption of moisture and HCs by the HCadsorbent 7 has been saturated when the humidity sensor 19 is brand-new.Since the parameter TMVR/TSH has been set depending on the resistance ofthe label resistive element 21, it matches the characteristics of theindividual unit of the humidity sensor 19.

If TM<TMVR/TSH in STEP23, then the deterioration evaluating device 17puts the processing sequence shown in FIG. 9 in the present cycle timeto an end. Therefore, until TM≧TMVR/TSH, i.e., until the engineoperation elapsed time TM reaches the time represented by the value ofthe parameter TMVR/TSH, the decision in STEP23 is made in each cycletime of the processing of the deterioration evaluating device 17. IfTM≧TMVR/TSH, then the deterioration evaluating device 17 acquires thedetected data of the present output voltage VRHUM of the humidity sensor19, i.e., the data of the output voltage VRHUM at the time the engineoperation elapsed time TM reaches the time represented by the value ofthe parameter TMVR/TSH, in STEP24. Thereafter, the deteriorationevaluating device 17 determines a value (=VRHUM−VRHUM/INI) which isproduced by subtracting the value of the parameter VRHUM/INI setdepending on the resistance of the label resistive element 21 (dependingon the characteristics of the individual unit of the humidity sensor 19)in the processing sequence shown in FIG. 4, from the acquired presentvalue of the output voltage VRHUM, as a characteristic change parameterVRHUMCH representative of a characteristic change of the humidity sensor19 in STEP25. The data of the output voltage VRHUM of the humiditysensor 19 acquired in STEP24 corresponds to characteristic changedetecting output data according to the present invention.

Referring to FIG. 7, the predetermined value VRHUM/INI is signified as areference value of the output voltage VRHUM at the time the outputvoltage VRHUM of the humidity sensor 19 reaches a value (a substantiallyconstant value) corresponding to the inherent humidity of the exhaustgas when the humidity sensor 19 is brand-new. Since the parameterVRHUM/INI has been set depending on the resistance of the labelresistive element 21, it matches the characteristics of the individualunit of the humidity sensor 19. Therefore, when the humidity sensor 19is brand-new (the curve a in FIG. 7), the characteristic changeparameter VRHUMCH is “0” irrespective of the individual unit of thehumidity sensor 19. If the humidity sensor 19 is deteriorated and itscharacteristics are changed (the curve b in FIG. 7), VRHUMCH>0 as shownin FIG. 7.

After having determined the characteristic change parameter VRHUMCH asdescribed above, the deterioration evaluating device 17 determines acorrective quantity COR/TMTRS for correcting the value of thedeterioration evaluating parameter TMTRS/PM from the characteristicchange parameter VRHUMCH based on a predetermined data table shown inFIG. 8 in STEP26.

The corrective quantity COR/TMTRS corrects the deterioration evaluatingparameter TMTRS/PM by being subtracted from the deterioration evaluatingparameter TMTRS/PM. The value of the characteristic change parameterVRHUMCH is substantially “0” if the humidity sensor 19 is brand-new ornearly brand-new. However, if the humidity sensor 19 is deteriorated toa certain extent, then the value of the characteristic change parameterVRHUMCH increases as the humidity sensor 19 is more deteriorated. Asdescribed above, when the humidity sensor 19 is deteriorated, the timing(the changing timing) at which the output voltage VRHUM of the humiditysensor 19 starts to decrease monotonously due to the saturation of theadsorption of moisture by the HC adsorbent 7 becomes later than if thehumidity sensor 19 is normal (if the humidity sensor 19 is brand-new).Therefore, the adsorption saturation timing detected in STEP18 becomeslater. Consequently, the data table shown in FIG. 8 is determined suchthat the corrective quantity COR/TMTRS is COR/TMTRS=0 if the value ofthe characteristic change parameter VRHUMCH is of a sufficiently smallvalue (VRHUMCH≦CH0 in FIG. 8). The data table shown in FIG. 8 is alsodetermined such that if the value of the characteristic change parameterVRHUMCH is large to a certain extent (VRHUMCH>CH0 in FIG. 8), thecorrective quantity COR/TMTRS is of a larger value as the characteristicchange parameter VRHUMCH is larger. The corrective quantity COR/TMTRSbasically serves to correct the value of the deterioration evaluatingparameter TMTRS/PM which is obtained when the humidity sensor 19 suffersa characteristic change due to its deterioration (the curve b in FIG.7), into the deterioration evaluating parameter TMTRS/PM which isobtained when the humidity sensor 19 brand-new (the curve a in FIG. 7).

After having determined the corrective quantity COR/TMTRS, thedeterioration evaluating device 17 subtracts the corrective quantityCOR/TMTRS from the value of the deterioration evaluating parameterTMTRS/PM obtained in STEP20, thus correcting the deteriorationevaluating parameter TMTRS/PM in STEP27. Since the deteriorationevaluating parameter TMTRS/PM thus determined has been corrected by thecorrective quantity COR/TMTRS depending on the characteristic changeparameter VRHUMCH, it has been compensated for the characteristic changeof the humidity sensor 19 due to its deterioration. Furthermore, in thiscase, because the output voltage VRHUM of the humidity sensor 19corresponding to the characteristic change parameter VRHUMCH is theoutput voltage VRHUM at the time when the time represented by theparameter TMVR/TSH set to cause the engine operation elapsed time TM tomatch the characteristics of the individual unit of the humidity sensor19 has elapsed, the output voltage VRHUM has also been compensated forthe characteristic variation of the individual unit of the humiditysensor 19. The characteristic change parameter VRHUMCH is produced bysubtracting the parameter VRHUM/INI as the reference value of the outputvoltage VRHUM of the individual unit of the humidity sensor 19 from theoutput voltage VRHUM of the humidity sensor 19 which corresponds to thecharacteristic change parameter VRHUMCH. Therefore, the deteriorationevaluating parameter TMTRS/PM obtained in STEP27 depends on thedeteriorated state of the HC adsorbent 7 irrespective of thecharacteristic change of the humidity sensor 19 due to its deteriorationand the characteristic variation of the individual unit of the humiditysensor 19. Since the corrective quantity COR/TMTRS is set to “0” whenthe characteristic change parameter VRHUMCH is smaller than thepredetermined value CH0 (see FIG. 8), the deterioration evaluatingparameter TMTRS/PM is not virtually corrected (is prohibited from beingcorrected) when VRHUMCH<CH0 (the characteristic change detected by thehumidity sensor 19 is sufficiently small).

Then, the deterioration evaluating device 17 compares the deteriorationevaluating parameter TMTRS/PM corrected as described above with thedeterioration evaluating threshold TRSTMDT that has been set dependingon the initial engine temperature TW of the engine 1 in the processingsequence shown in FIG. 4 in STEP28. If TMTRS/PM>TRSTMDT, then thedeterioration evaluating device 17 judges that the HC adsorbent 7 is inthe non-deteriorated state, and sets the value of a flag F/TRSDT to “0”in STEP29. Then, the deterioration evaluating device 17 resets the valueof the flag F/MCND to “0” in step S32, and thereafter puts theprocessing sequence shown in FIG. 6 to an end. The flag F/TRSDT set inSTEP29 is a flag which is “0” when the HC adsorbent 7 is in thenon-deteriorated state and “1” when the HC adsorbent 7 is in thedeterioration-in-progress state.

If TMTRS/PM≦TRSTMDT in STEP28, then the deterioration evaluating device17 judges that the HC adsorbent 7 is in the deterioration-in-progressstate, and sets the value of the flag F/TRSDT to “1” in STEP30. Then,the deterioration evaluating device 17 controls the deteriorationindicator 18 to indicate that the HC adsorbent 7 is in thedeterioration-in-progress state in STEP31. Thereafter, the deteriorationevaluating device 17 resets the value of the flag F/MCND to “0” inSTEP32, and thereafter puts the processing sequence shown in FIG. 6 toan end.

In the present embodiment described above, inasmuch as the deterioratedstate of the HC adsorbent 7 is evaluated based on the deteriorationevaluating parameter TMTRS/PM which has been determined to compensatefor a characteristic change of the humidity sensor 19 due to itsdeterioration and a characteristic variation of the individual unit ofthe humidity sensor 19, the deteriorated state of the HC adsorbent 7 canbe evaluated appropriately irrespective of the characteristic change ofthe humidity sensor 19 due to its deterioration and the characteristicvariation of the individual unit of the humidity sensor 19. Since thecharacteristic change of the humidity sensor 19 due to its deteriorationand the characteristic variation of the individual unit of the humiditysensor 19 can be compensated for, the requirements for the steadiness ofthe characteristics of individual units of the humidity sensor 19 andthe uniformity of the characteristics of each of the individual units ofthe humidity sensor 19 are lessened. Therefore, the costs needed todevelop and manufacture the humidity sensor 19 can be reduced.

In the embodiment described above, the deteriorated state of the HCadsorbent 7 is evaluated. However, the present invention is alsoapplicable to the monitoring of the adsorption of HCs or moisture by theHC adsorbent 7 to determine, e.g., whether the adsorption of moistureand HCs by the HC adsorbent 7 has been saturated or not.

In the above embodiment, the deterioration evaluating parameter TMTRS/PMis corrected depending on the characteristic change parameter VRHUMCH.However, the deterioration evaluating threshold TRSTMDT, rather than thedeterioration evaluating parameter TMTRS/PM, may be corrected. In such acase, the deterioration evaluating threshold TRSTMDT may be corrected byadding the corrective quantity COR/TMTRS to the deterioration evaluatingthreshold TRSTMDT, and the corrected deterioration evaluating thresholdTRSTMDT may be compared with the deterioration evaluating parameterTMTRS/PM (which is obtained in STEP20 shown in FIG. 6).

In the above embodiment, if the value of the characteristic changeparameter VRHUMCH becomes greater than a suitable upper limit value (ifVRHUMCH becomes excessively large), then the humidity sensor 19 maypossibly be excessively deteriorated or may possibly suffer a failure.In this case, the substantial evaluation of the deteriorated state ofthe HC adsorbent 7 (the processing from STEP26 shown in FIG. 6) may notbe performed.

In the above embodiment, the engine operation elapsed time TM is used asdata representative of the integrated amount of moisture given to the HCadsorbent 7 after the engine 1 has started to operate. However, theintegrated value of the amount of fuel supplied from the start ofoperation of the engine 1 (which may be a command value generated by theECU 16), or the integrated value of a detected or estimated value of theamount of intake air from the start of operation of the engine 1 may beused as data representative of the integrated amount of moisture. Inthis case, the engine 1 may not be idling after it has started tooperate.

A second embodiment of the present invention will be described belowwith reference to FIGS. 1 through 3 and FIGS. 9 through 14. The presentembodiment differs from the first embodiment only as to a portion of theapparatus arrangement and the processing sequence of the deteriorationevaluating device. Therefore, those parts of the second embodiment whichare identical to those of the first embodiment are denoted by referencecharacters that are identical to those of the first embodiment, and willnot be described in detail below.

According to the present embodiment, as shown in FIG. 1, the apparatushas, in addition to the structural details of the first embodiment, anair-fuel ratio sensor 23 mounted on the first exhaust pipe 5 a upstreamof the catalytic converter 6 for detecting the air-fuel ratio of theair-fuel mixture which has been combusted by the engine 1, and anatmospheric temperature sensor 24 for detecting the temperature of theatmosphere as the temperature outside of the engine 1 and its exhaustsystem (the exhaust pipe 5, etc.). The other structural details of theapparatus are identical to those of the first embodiment.

According to the present embodiment, the deterioration evaluating device17 performs a flowchart shown in FIG. 9 when the engine 1 starts tooperate. The processing sequence shown in FIG. 9 is different from theprocessing sequence shown in FIG. 4 only as to the processing in STEP47,STEP48. The processing in STEP41 through STEP46 and STEP49 is identicalto the processing in STEP1 through STEP6 and STEP10 shown in FIG. 4.

The processing that is different from the first embodiment will bedescribed below. In STEP47 (which corresponds to STEP8 in FIG. 4), thedeterioration evaluating device 17 sets the value of the parameterVRHUM/INI (see FIG. 7) for compensating for the effect of characteristicvariations of individual units of the humidity sensor 19 in a process,to be described later on, for evaluating the deterioration of the HCadsorbent 7, from the detected data of the resistance LBLR of the labelresistive element 21 which has been acquired in STEP45. The significanceof VRHUM/INI and the manner in which it is set are identical to those ofthe first embodiment. The parameters TM/SH, TMVR/TSH described in thefirst embodiment are not set in the present embodiment. As describedlater on, according to the present embodiment, the parameter TM/SH isset to a predetermined fixed value. In the present embodiment, aparameter TMVR/TSH2 (see FIG. 7) set to a predetermined fixed value isused instead of the parameter TMVR/TSH.

In STEP48 (which corresponds to STEP9 shown in FIG. 4) following STEP47,the deterioration evaluating device 17 initializes, to “0”, the value ofthe timer TM (count-up timer) which measures a period of time that haselapsed from the start of operation of the engine 1, and alsoinitializes the flag F/RST (the flag indicative of whether the detectionof the timing at which the output voltage VRHUM of the humidity sensor19 changes to the tendency to decrease monotonously is finished or not)to “0”.

According to the present embodiment, the deterioration evaluating device17 additionally initializes the value of a flag F/VROFF, to be describedlater on, to According to the present embodiment, the processingcorresponding to STEP7 shown in FIG. 4 according to the first embodiment(the setting of the deterioration evaluating threshold TRSTMDT) is notperformed. This is because the parameter used to evaluate thedeteriorated state of the HC adsorbent 7 in the present embodimentdiffers from the parameter used in the first embodiment.

In the processing sequence shown in FIG. 9, as with the firstembodiment, in addition to determining the conditions in STEP1 andSTEP3, the deterioration evaluating device 17 may determine whethersoaking has been completed or not for setting the value of the flagF/MCND, and may set the value of the flag F/MCND to “0” if soaking hasnot been completed.

After having performed the processing sequence shown in FIG. 9 when theengine 1 starts to operate, the deterioration evaluating device 17performs a processing sequence shown in a flowchart of FIG. 10 in apredetermined cycle time while the engine 1 is in operation. Theprocessing sequence shown in FIG. 10 is basically a process of graspinga characteristic change of the humidity sensor 19 based on thetransitional characteristics of the output voltage VRHUM of the humiditysensor 19 after the engine 1 has started to operate, as described abovewith reference to FIG. 7, and has many processing details common to theprocessing sequence shown in FIG. 6 according to the first embodiment.The processing sequence shown in FIG. 10 according to the presentembodiment is performed regardless of whether the engine 1 is idling ornot.

More specifically, the deterioration evaluating device 17 determines thevalue of the flag F/MCND set in the processing sequence shown in FIG. 9in STEP51 as with the first embodiment. If F/MCND=0, then since it isnot appropriate in detecting a characteristic change of the humiditysensor 19, the deterioration evaluating device 17 puts the processingsequence shown in FIG. 10 to an end.

If F/MCND=1 in STEP51, then the deterioration evaluating device 17determines the value of a flag F/VROFF in STEP52. The flag F/VROFF is“1” if the data of a characteristic change parameter VRHUMOFF, to bedescribed later on, has been acquired, and “0” if the data of thecharacteristic change parameter VRHUMOFF has not been acquired. When theengine 1 has started to operate, the characteristic change parameterVRHUMOFF has been initialized to “0” in STEP48 shown in FIG. 9. Theprocessing sequence shown in FIG. 10 is basically a process of acquiringthe data of the characteristic change parameter VRHUMOFF. If VRHUMOFF=1in STEP52, then the processing sequence shown in FIG. 10 is put to anend.

If F/VROFF=0 in STEP52, then the deterioration evaluating device 17performs the same processing as the processing in STEP12 through STEP22according to the first embodiment in STEP53 through STEP62.Specifically, the deterioration evaluating device 17 sequentiallydetermines the maximum output parameter VRHUM/MAX (see FIG. 7) as themaximum value of the output voltage VRHUM of the humidity sensor 19 inSTEP53 through STEP58 after the engine 1 has started to operate. Thisprocessing is exactly the same as with the first embodiment.

If the engine operation elapsed time TM when the output voltage VRHUM ofthe humidity sensor 19 becomes VRHUM<VRHUM/MAX−VRHUM/JUD (YES in STEP59,the time t1 or t2 in FIG. 7) falls in the time of the parameter TM/SHafter the engine 1 has started to operate (YES in STEP60), then thevalue of the flag F/RST is set to “1” in STEP61. If the engine operationelapsed time TM exceeds the time of the parameter TM/SH (NO in STEP60),then the value of the flag F/RST is set to “0” in STEP62. In the presentembodiment, the value of the parameter TM/SH is basically set such thatthe output voltage VRHUM of the humidity sensor 19 becomesVRHUM<VRHUM/MAX−VRHUM/JUD within the time of the parameter TM/SHirrespective of characteristic variations of individual units of thehumidity sensor 19.

Since the processing sequence shown in FIG. 10 according to the presentembodiment is performed regardless of whether the engine 1 is idling ornot, the engine operation elapsed time TM does not necessarilycorrespond to the integrated amount of moisture supplied to the HCadsorbent 7. According to the present embodiment, since the parameterfor evaluating the deteriorated state of the HC adsorbent 7 is differentfrom the parameter in the first embodiment, the value of the engineoperation elapsed time TM when the output voltage VRHUM of the humiditysensor 19 becomes VRHUM<VRHUM/MAX−VRHUM/JUD within the time of theparameter TM/SH is not stored. That is, the processing whichcorresponding to STEP20 shown in FIG. 6 is not performed in the presentembodiment. The processing in STEP53 through STEP62 according to thepresent embodiment differs from the processing in STEP12 through STEP22shown in FIG. 6 according to the first embodiment only as to this point.

If the value of the flag F/RST is set in STEP61, then F/RST=1 in thedecision processing in STEP54. At this time, the deteriorationevaluating device 17 compares the engine operation elapsed time TM witha predetermined value TMVR/TSH2 in STEP63. The predetermined valueTMVR/TSH2 corresponds to the parameter TMVR/TSH in the first embodiment,and is a predetermined fixed value according to the present embodiment.More specifically, as shown in FIG. 7, the predetermined value TMVR/TSH2is determined by way of experimentation or the like such that when theengine operation elapsed time TM reaches the predetermined valueTMVR/TSH2, the humidity of the exhaust gas detected by the humiditysensor 19 is a steady humidity (a substantially constant humidityinherent in the exhaust gas as described above) after the adsorption ofmoisture by the HC adsorbent 7 is saturated. According to the presentembodiment, therefore, the predetermined value TMVR/TSH2 is set to avalue which is sufficiently larger than the parameter TMVR/TSH used inthe first embodiment.

Then, the deterioration evaluating device 17 acquires the detected dataof the present output voltage VRHUM of the humidity sensor 19, i.e., thedata of the output voltage VRHUM at the time the engine operationelapsed time TM reaches the time represented by the value of thepredetermined parameter TMVR/TSH2, in STEP64. Thereafter, thedeterioration evaluating device 17 determines a value (=VRHUM−VRHUM/INI)which is produced by subtracting the value of the parameter VRHUM/INIset depending on the resistance of the label resistive element 21(depending on the characteristics of the individual unit of the humiditysensor 19) in the processing sequence shown in FIG. 4, from the acquiredoutput voltage VRHUM, as a characteristic change parameter VRHUMOFFrepresentative of a characteristic change of the humidity sensor 19 inSTEP65. The characteristic change parameter VRHUMOFF thus determined issignified as an offset voltage produced by the deterioration of thehumidity sensor 19, as shown in FIG. 7. As described above in the firstembodiment, the parameter VRHUM/INI is set depending on the resistanceof the label resistive element 21 (STEP47 in FIG. 9), and matches thecharacteristics of the individual unit of the humidity sensor 19.Therefore, when the humidity sensor 19 is brand-new, VRHUMOFF=0 (see thecurve a in FIG. 7) irrespective of characteristic variations ofindividual units of the humidity sensor 19. As the humidity sensor 19 isprogressively deteriorated, the value of VRHUMOFF becomes larger (seethe curve b in FIG. 7) in a pattern that is substantially constantregardless of characteristic variations of individual units of thehumidity sensor 19. That is, the characteristic change parameterVRHUMOFF represents the degree to which the humidity sensor 19 isdeteriorated regardless of characteristic variations of individual unitsof the humidity sensor 19.

After having determined the characteristic change parameter VRHUMOFF,the deterioration evaluating device 17 compares the value of thecharacteristic change parameter VRHUMOFF with a predetermined valueVRHUM/DJUD in STEP66. The predetermined value VRHUM/DJUD is a positivevalue close to “0”. If VRHUMOFF≦VRHUM/DJUD, i.e., if the characteristicchange parameter VRHUMOFF is sufficiently small (if the deterioration ofthe humidity sensor 17 has not essentially been in progress), thedeterioration evaluating device 17 forcibly sets the value of VRHUMOFFto “0” in STEP67. This is not to correct the value of a deteriorationevaluating parameter according to the present embodiment if thedeterioration of the humidity sensor 19 is sufficiently small. IfVRHUMOFF>VRHUM/DJUD, then the value of VRHUMOFF is maintained as it is.

Then, the deterioration evaluating device 17 sets the value of the flagF/VROFF indicative of whether the data of the characteristic changeparameter VRHUMOFF has been acquired or not, to “1” in STEP68, andthereafter compares the value of the characteristic change parameterVRHUMOFF with a predetermined value VRHUMOFF/JUD in STEP69. Thepredetermined value VRHUMOFF/JUD is signified as an upper limit for thecharacteristic change parameter VRHUMOFF capable of appropriatelyevaluating the deteriorated state of the HC adsorbent 7 using the outputvoltage VRHUM of the humidity sensor 19 as described later on. IfVRHUMOFF≧VRHUMOFF/JUD, then the deterioration evaluating device 17 setsthe value of a flag F/HUMNG to “0” in STEP70 a, and puts the processingsequence shown in FIG. 10 to an end. If VRHUMOFF<VRHUMOFF/JUD, then thedeterioration evaluating device 17 sets the value of the flag F/HUMNG to“1” in STEP70 b, and puts the processing sequence shown in FIG. 10 to anend. The flag F/HUMNG is “1” if the humidity sensor 19 is excessivelydeteriorated and is in a state incapable of appropriately evaluating thedeteriorated state of the HC adsorbent 7, and “0” if the humidity sensor19 is not excessively deteriorated and is not in a state incapable ofappropriately evaluating the deteriorated state of the HC adsorbent 7.

The value of the characteristic change parameter VRHUMOFF and the valuesof the flag F/VROFF and the value of the flag F/FUMNG are stored in anonvolatile memory such as an EEPROM or the like (not shown) or a memorythat is energized at all times by a battery or the like (not shown) whenthe engine 1 is not in operation, so that those values will not be lostwhile the engine 1 is not in operation.

While the engine 1 is in operation, the deterioration evaluating device17 performs a processing sequence according to a flowchart shown in FIG.11 in a predetermined cycle time in addition to the processing sequenceshown in FIG. 10. According to the processing sequence shown in FIG. 11,the deterioration evaluating device 17 acquires present detected valuedata of the engine temperature TW of the engine 1 from the ECU 16 anddetected value data of the air-fuel ratio KACT of the air-fuel mixturewhich has been combusted by the engine 1 (hereinafter referred to as“air-fuel ratio KACT of the engine 1”) based on the present output ofthe air-fuel sensor 23 shown in FIG. 1 in STEP71.

Then, the deterioration evaluating device 17 determines whether thedetected value (present value) of the engine temperature TW of theengine 1 is higher than a predetermined value TWHOT or not in STEP 72for thereby determining whether the engine 1 has been warmed upsufficiently or not. The predetermined value TWHOT is set to 85° C., forexample. When the engine temperature TW is higher than the predeterminedvalue TWHOT (TW>TWHOT), the catalytic converter 6 has basically beenwarmed and activated sufficiently, and the HC adsorbent 7 of the exhaustgas purifier 8 has been warmed to a temperature capable of releasing theadsorbed HCs.

If TW≦TWHOT in STEP 72, then since given conditions for appropriatelyevaluating a deteriorated state of the HC adsorbent 7, i.e., conditionsregarding the engine temperature TW and the air-fuel ratio KACT duringoperation of the engine 1, have not been satisfied, the deteriorationevaluating device 17 sets a flag F/CND to “0” in STEP79, and then putsthe processing sequence in FIG. 11 in the present control cycle to anend. The flag F/CND is a flag used when the deterioration evaluatingdevice 17 executes a process of evaluating a deteriorated state of theHC adsorbent 7 while the engine 1 is being shut off, as described lateron. The flag F/CND has an initial value of “0” at the time the engine 1starts to operate.

If TW>TWHOT in STEP 72, then the deterioration evaluating device 17determines whether the detected value (present value) of the air-fuelratio KACT of the engine 1 falls in a predetermined range, i.e., a rangeof AFL<KACT<AFH, in the vicinity of the stoichiometric air-fuel ratio ornot in STEP 73. The lower-limit value AFL of the range represents anair-fuel ratio slightly leaner than the stoichiometric air-fuel ratio,and the upper-limit value AFH of the range represents an air-fuel ratioslightly richer than the stoichiometric air-fuel ratio.

If the detected value of the air-fuel ratio KACT of the engine 1 fallsoutside of the predetermined range, i.e., if KACT≦AFL or KACT≧AFH, inSTEP 73, then the deterioration evaluating device 17 initializes thecount value CDTM of a count-down timer for measuring a period of time inwhich the air-fuel ratio KACT is continuously kept in the abovepredetermined range, to a given initial value CDTM0 in STEP 78. Then,the deterioration evaluating device 17 sets the flag F/CND to “0” inSTEP79, after which the processing sequence shown in FIG. 11 in thepresent control cycle is ended.

If AFL<KACT<AFH in STEP 73, then the deterioration evaluating device 17judges the value (present value) of the flag F/CND in STEP74. IfF/CND=1, then the deterioration evaluating device 17 puts the processingsequence shown in FIG. 11 in the present control cycle to an end. IfF/CND=0, then the deterioration evaluating device 17 counts down thecount value CDTM of the count-down timer by a given value Δtm in STEP75.The deterioration evaluating device 17 determines in STEP76 whether ornot the count value CDTM is “0” or smaller, i.e., whether the statewhere AFL<KACT<AFH or the state where the air-fuel ratio KACT of theengine 1 is close to the stoichiometric air-fuel ratio has continued forat least a given period of time corresponding to the initial value CDTM0of the count value CDTM of the count-down timer or not.

If CDTM>0, then the deterioration evaluating device 17 sets the flagF/CND to “0” in STEP79, after which the processing sequence in thepresent control cycle is ended. If CDTM≦0, then since the conditions forappropriately evaluating a deteriorated state of the HC adsorbent 7,i.e., the conditions regarding the engine temperature TW and theair-fuel ratio KACT during operation of the engine 1, have beensatisfied, the deterioration evaluating device 17 sets the flag F/CND to“1” in STEP77. Thereafter, the processing sequence in the presentcontrol cycle is ended.

According to the processing sequence shown in FIG. 11 which has beendescribed above, if the engine temperature TW of the engine 1 is higherthan the predetermined value TWHOT and the air-fuel ratio KACT of theengine 1 has continuously been kept close to the stoichiometric air-fuelratio for at least the period of time corresponding to the initial valueCDTM0 of the count-down timer, then the flag F/CND is set to “1”. If theengine temperature TW of the engine 1 is lower than the predeterminedvalue TWHOT or the air-fuel ratio KACT falls out of a given range closeto the stoichiometric air-fuel ratio due to a temporary disturbance or afuel-cutoff operation of the engine 1, or if the air-fuel ratio KACT hasnot continuously been kept close to the stoichiometric air-fuel ratiofor the above period of time although the air-fuel ratio KACT is in therange close to the stoichiometric air-fuel ratio, then the flag F/CND isset to “0”.

The processing sequence according to the flowchart shown in FIG. 11 iscarried out only while the engine 1 is in operation. The value of theflag F/CND is determined while the engine 1 is being shut off. Toprevent the value of the flag F/CND from being lost while the engine 1is being shut off, the deterioration evaluating device 17 stores thevalue of the flag F/CND in a nonvolatile memory such as an EEPROM or thelike (not shown) or a memory that is energized at all times by a batteryor the like (not shown). Therefore, while the engine 1 is being shutoff, the flag F/CND is set to “1” only if the engine temperature TW ofthe engine 1 is higher than the predetermined value TWHOT and theair-fuel ratio KACT of the engine 1 has continuously been kept close tothe stoichiometric air-fuel ratio for at least the period of timecorresponding to the initial value CDTM0 of the count-down timerimmediately before the engine 1 is shut off.

The deterioration evaluating device 17 which performs the processingsequence according to the flowcharts shown in FIGS. 10 and 11 while theengine 1 is in operation performs a processing sequence according to aflowchart shown in FIG. 12 to evaluate a deteriorated state of the HCadsorbent 7 at a given timing while the engine 1 is not in operation.Prior to describing the processing sequence according to the flowchartshown in FIG. 12 (hereinafter referred to as “deterioration evaluatingprocess”), time-dependent changes of the output voltage VRHUM of thehumidity sensor 19 (which is signified as the detected value of therelative humidity near the HC adsorbent 7) and time-dependent changes ofthe engine temperature TW of the engine 1 during shutdown of the engine1, and a basic concept of a process of evaluating a deteriorated stateof the HC adsorbent 7 according to the present embodiment will first bedescribed below with reference to FIG. 13.

FIG. 13 shows, in an upper section thereof, curves a, b, c representingtime-depending changes of the detected value of the relative humidityVHUM from the humidity sensor 19 after the engine 1 is shut off, thecurves a, b, c corresponding respectively to a brand-new HC adsorbent 7,a mediumly deteriorated HC adsorbent 7, and a largely deteriorated HCadsorbent 7. FIG. 13 also shows, in a lower section thereof, a curve drepresenting time-depending changes of the detected value of the enginetemperature TW after the engine 1 is shut off.

The relative humidity VHUM near the HC adsorbent 7 temporarily increasesimmediately after the engine 1 is shut off because the saturated watervapor pressure is lowered due to a reduction in the temperature of theexhaust gas purifier 8, etc. Therefore, the output voltage VRHUM of thehumidity sensor 19 according to the present embodiment, which has anegative humidity coefficient as shown in FIG. 2, temporarily decreasesimmediately after the engine 1 is shut off, as indicated by the curvesa, b, c in the upper section of FIG. 13. When the temperature of the HCadsorbent 7 of the exhaust gas purifier 8 drops to a value for adsorbingmoisture as well as HCs in the exhaust gas, since the HC adsorbent 7starts to adsorbs moisture in the exhaust gas present around the HCadsorbent 7, the relative humidity VHUM near the HC adsorbent 7 changesfrom the tendency to increase to a tendency to decrease. As a result, asindicated by the curves a, b, c in the upper section of FIG. 13, theoutput voltage VRHUM of the humidity sensor 19 takes a minimum valueafter it has been temporarily reduced as described above, and thenincreases. The above increase and decrease of the output voltage VRHUMof the humidity sensor 19 do not occur instantaneously, but generallytake a time ranging from several tens seconds to several hours.

When the HC adsorbent 7 continuously adsorbs moisture until it issaturated, the relative humidity near the HC adsorbent 7 and hence theoutput voltage VRHUM of the humidity sensor 19 have minimumtime-dependent changes and become substantially constant for arelatively long period (a period Δtpx in FIG. 4). As the HC adsorbent 7is deteriorated to a larger extent (i.e., as its ability to absorb HCsand moisture is lowered to a larger extent), the maximum amount ofmoisture which can be adsorbed by the HC adsorbent 7 is smaller. Thelevel of the relative humidity as it is substantially constant near theHC adsorbent 7 is higher as the HC adsorbent 7 is deteriorated to alarger extent and lower as the HC adsorbent 7 is deteriorated to asmaller extent. Therefore, the output voltage VRHUM of the humiditysensor 19 at the time the relative humidity near the HC adsorbent 7 issubstantially constant is smaller as the HC adsorbent 7 is deterioratedto a larger extent and larger as the HC adsorbent 7 is deteriorated to asmaller extent, as indicated by the curves a, b, c in FIG. 13.

The output voltage VRHUM of the humidity sensor 19 is affected by thedeteriorated state thereof. If there is developed an offset voltagecorresponding to the characteristic change parameter VRHUMOFF due to itsdeterioration, the level of the output voltage VRHUM increases by theoffset voltage.

When the period of time that has elapsed after the engine 1 is shut off,i.e., the period of time that has elapsed during shutdown of the engine1, becomes sufficiently long (e.g., in the order of several tens ofhours), because a gas exchange between the interior of the exhaust pipe5 and the bypass exhaust passage 12 and the atmosphere graduallyprogresses, the relative humidity near the HC adsorbent 7 and hence theoutput voltage VRHUM of the humidity sensor 19 finally converge to avalue corresponding to the atmospheric humidity (ambient humidity)outside of the exhaust pipe 5 and the bypass exhaust passage 12, asindicated by right portions of the curves a, b, c in FIG. 13.

The timing when the period Δtpx (hereinafter referred to as “steadyhumidity period Δtpx”) in which the relative humidity near the HCadsorbent 7 (and hence the output voltage VRHUM of the humidity sensor19) is actually substantially constant starts after the engine 1 is shutoff depends on the temperature of the exhaust system (the exhaust gaspurifier 8, etc.), the atmospheric temperature TA, the volume of the HCadsorbent, etc. at the time the engine 1 is shut off. With the systemaccording to the present embodiment, the steady humidity period Δtpxstarts when about two through four hours have elapsed after the engine 1is shut off. The steady humidity period Δtpx ends depending on thestructure of the exhaust system from the exhaust gas purifier 8 to thedownstream end of the exhaust pipe 5. According to the presentembodiment, a catalytic converter and a muffler (silencer) or the like,which are not shown, are provided downstream of the exhaust gas purifier8, and the steady humidity period Δtpx ends when about 24 through 72hours, representing a period t/max in FIG. 13, have elapsed after theengine 1 is shut off.

As can be seen from the curve d in the lower section of FIG. 13, theengine temperature TW of the engine 1 gradually drops after the engine 1is shut off, and is finally converged to the atmospheric temperature TAdetected by the atmospheric temperature sensor 24. The temperature ofthe exhaust system of the engine 1, e.g., the temperature of the exhaustgas purifier 8, also basically drops according to the tendency of theengine temperature TW, and is finally converged to the atmospherictemperature TA. When the engine temperature TW and the temperature ofthe exhaust system of the engine 1, e.g., the exhaust gas purifier 8,etc., drop to a temperature equivalent to the atmospheric temperatureTA, since the saturated water vapor pressure present in the exhaustsystem becomes substantially constant, the relative humidity near the HCadsorbent 7 and hence the output voltage VRHUM of the humidity sensor 19basically become substantially constant.

As described above, the output voltage VRHUM of the humidity sensor 19during the steady humidity period Δtpx is substantially constant and thelevel of the substantially constant output voltage VRHUM depends on thedeteriorated state of the HC adsorbent 7. According to the presentembodiment, therefore, the output voltage VRHUM of the humidity sensor19 in the steady humidity period Δtpx while the engine 1 is being shutoff is used to evaluate whether the deteriorated state of the HCadsorbent 7 is the non-deteriorated state or thedeterioration-in-progress state. In the present embodiment, after agiven period of time t/min (see FIG. 13) has elapsed from the shutdownof the engine 1, the output voltage VRHUM of the humidity sensor 19 atthe time the engine temperature TW is substantially converged to theatmospheric temperature TA is used to evaluate the deteriorated state ofthe HC adsorbent 7. Stated otherwise, it is assumed that the period oftime in which the relative humidity near the HC adsorbent 7 issubstantially constant begins from the time when the predeterminedperiod of time t/min has elapsed and the engine temperature TW issubstantially converged to the atmospheric temperature TA, and thedeteriorated state of the HC adsorbent 7 is evaluated using the outputvoltage VRHUM of the humidity sensor 19 at the beginning of that periodof time. The predetermined period of time t/min is basically determinedsuch that the time when the period of time that has elapsed after theengine 1 is shut off reaches the predetermined period of time t/min ispresent in the steady humidity period Δtpx, and is set to two hours, forexample, according to the present embodiment.

On the basis of the foregoing description, the deterioration evaluatingprocess which is carried out by the deterioration evaluating device 17during shutdown of the engine 1 will be described below with referenceto FIG. 12.

The apparatus according to the present invention has a timer (not shown,hereafter referred to as “off timer”) for measuring a period of timethat elapses from the shutdown of the engine 1 and activating the ECU 16and the deterioration evaluating device 17 (with electric energysupplied from the non-illustrated battery) when the measured period oftime has reached a preset period of time. The deterioration evaluatingdevice 17 executes the deterioration evaluating process shown in FIG. 12only when the ECU 16 and the deterioration evaluating device 17 areactivated by the off timer while the engine 1 is being shut off. The offtimer is set to the predetermined period of time t/min (see FIG. 13)when the engine 1 is shut off. Therefore, the deterioration evaluatingprocess according to the flowchart shown in FIG. 12 is executed for thefirst time when the predetermined period of time t/min (two hours in thepresent embodiment) has elapsed after the shutdown of the engine 1 whilethe engine 1 is being shut off.

Specifically, the deterioration evaluating process is carried out asfollows: As shown in FIG. 12, the deterioration evaluating device 17determines whether the release of the HCs adsorbed by the HC adsorbent 7during the preceding operation of the engine 1 is completed or not basedon the value of a flag F/HCPG in STEP81. The flag F/HCPG is “1” if therelease of the HCs adsorbed by the HC adsorbent 7 is completed and “0”if the release of the HCs adsorbed by the HC adsorbent 7 is notcompleted. The flag F/HCPG is set as described above with respect toSTEP1 shown in FIG. 4 according to the first embodiment.

If F/HCPG=0 in STEP81, since the release of the HCs and the moistureadsorbed by the HC adsorbent 7 during the preceding operation of theengine 1 is not completed (the HC adsorbent 7 has already adsorbed themoisture when the engine 1 is shut off), the deterioration evaluatingdevice 17 sets the value of an evaluation result parameter SKrepresenting an evaluation result of the deteriorated state of the HCadsorbent 7 to “0” in STEP91. Thereafter, the deterioration evaluatingdevice 17 puts the deterioration evaluating process to an end. When theevaluation result parameter SK is “0”, it indicates that the evaluationof the deteriorated state of the HC adsorbent 7 is not determined. Whenthe evaluation result parameter SK is “1”, it indicates that the HCadsorbent 7 is in the non-deteriorated state. When the evaluation resultparameter SK is “2”, it indicates that the HC adsorbent 7 is in thedeterioration-in-progress state.

If F/HCPG=1 in STEP81, i.e., if the release of the HCs and the moistureadsorbed by the HC adsorbent 7 during the preceding operation of theengine 1 is completed, the deterioration evaluating device 17 determinesthe value of the flag F/CND set according to the processing sequence inFIG. 11 in the preceding operation of the engine 1 in STEP82. IfF/CND=0, i.e., if the engine temperature TW is lower than thepredetermined value TWHOT immediately before the engine 1 is shut off orif the state in which the air-fuel ratio KACT immediately before theinternal combustion engine 1 is shut off is kept close to thestoichiometric air-fuel ratio has not continued for the predeterminedperiod of time, then the deterioration evaluating device 17 regardsthese conditions as inappropriate for finalizing the evaluation of thedeteriorated state of the HC adsorbent 7, and sets the evaluation resultparameter SK to “0” in STEP91. Thereafter, the deterioration evaluatingdevice 17 puts the deterioration evaluating process to an end.

If F/CND=1 in STEP82, i.e., if the engine temperature TW is higher thanthe predetermined value TWHOT immediately before the engine 1 is shutoff and if the state in which the air-fuel ratio KACT immediately beforethe engine 1 is shut off is kept close to the stoichiometric air-fuelratio has continued for at least the predetermined period of time, thenthe deterioration evaluating device 17 successively determines thevalues of the flags F/VROFF, F/HUMNG that are set according to theprocessing sequence shown in FIG. 10 in STEP83 and STEP84. If F/VROFF=0,then since the deterioration evaluating device 17 has not properlyacquired the data of the characteristic change parameter VRHUMOFF, thedeterioration evaluating device 17 sets the value of the evaluationresult parameter SK to “0” in STEP91. Thereafter, the deteriorationevaluating device 17 puts the deterioration evaluating process to anend. Even if F/VROFF=1, if F/HUMNG=1, then since the value of thecharacteristic change parameter VRHUMOFF is excessively large (thehumidity sensor 19 is excessively deteriorated), the deteriorationevaluating device 17 judges that it is difficult to properly evaluatethe deteriorated state of the HC adsorbent 7, and sets the value of theevaluation result parameter SK to “0” in STEP91. Thereafter, thedeterioration evaluating device 17 puts the deterioration evaluatingprocess to an end (the evaluation of the deteriorated state of the HCadsorbent 7 is prohibited).

If F/VROFF=1 and F/HUMNG=0 (YES in STEP84), then the deteriorationevaluating device 17 acquires detected value data of the present enginetemperature TW of the engine 1 from the ECU 16, acquires detected valuedata of the present atmospheric temperature TA (the temperature outsideof the exhaust system including the exhaust gas purifier 8, etc.) fromthe atmospheric temperature sensor 24, and detected data of the presentoutput voltage VRHUM of the humidity sensor 21 in STEP85.

Then, the deterioration evaluating device 17 determines whether thedifference (TW−TA) between the present engine temperature TW and theatmospheric temperature TA is smaller than a predetermined value DT ornot in STEP86. The predetermined value DT is a sufficiently smallpositive value. If TW−TA<DT, then it means that the engine temperatureTW has dropped to a temperature (substantially constant) substantiallyequal to the atmospheric temperature TA and the temperature near the HCadsorbent 7 has dropped to a temperature (substantially constant)substantially equal to the atmospheric temperature TA. In STEP86, thedeterioration evaluating device 17 uses the engine temperature TW inorder to recognize the temperature near the HC adsorbent 7. However, ifthe temperature near the HC adsorbent 7 is directly detected by atemperature sensor, then the deterioration evaluating device 19 may usethe thus detected temperature near the HC adsorbent 7 instead of theengine temperature TW.

If TW−TA<DT in STEP86, i.e., if the engine temperature TW (and thetemperature near the HC adsorbent 7) is substantially converged to theatmospheric temperature and is substantially constant, the outputvoltage VRHUM of the humidity sensor 19 (the relative humidity near theHC adsorbent 7) is substantially constant for certain. The deteriorationevaluating device 17 now determines a predetermined threshold VRHUM/JUD2for evaluating the deteriorated state of the HC adsorbent 7 from apredetermined data table shown in FIG. 14 in STEP87. The thresholdVRHUM/JUD2 is a threshold to be compared with a value that is producedby subtracting an offset voltage of the value of the characteristicchange parameter VRHUMOFF from the output voltage VRHUM of the humiditysensor 19 (acquired in STEP85). As shown in FIG. 14, the thresholdVRHUM/JUD2 is set depending on the engine temperature TW such that it issmaller as the engine temperature TW (which represents the temperatureof the HC adsorbent 7) is lower. The threshold VRHUM/JUD2 is setdepending on the engine temperature TW (the temperature of the HCadsorbent 7) because the HC adsorbent 7 adsorbs more moisture as itstemperature is lower.

Then, the deterioration evaluating device 17 compares a value(=VRHUM−VRHUMOFF) that is produced by subtracting the value of thecharacteristic change parameter VRHUMOFF obtained in the processingsequence shown in FIG. 11 during the preceding operation of the engine 1from the present output voltage VRHUM (the deterioration evaluatingparameter) of the humidity sensor 19 acquired in STEP85, i.e., a valuethat is produced by correcting the output voltage VRHUM as thedeterioration evaluating parameter depending on the characteristicchange parameter VRHUMOFF, with the threshold VRHUM/JUD2 determined inSTEP87 in STEP88. If VRHUM−VRHUMOFF≧VRHUM/JUD2, then the deteriorationevaluating device 17 judges that the HC adsorbent 7 is in thenon-deteriorated state (corresponding to the curves a, b shown in FIG.13), and sets the evaluation result parameter SK to “1” in STEP89.Thereafter, the deterioration evaluating device 17 puts thedeterioration evaluating process to an end. IfVRHUM−VRHUMOFF<VRHUM/JUD2, then the deterioration evaluating device 17judges that the HC adsorbent 7 is in the deterioration-in-progress state(corresponding to the curve c shown in FIG. 13), and sets the evaluationresult parameter SK to “2” in STEP90. Thereafter, the deteriorationevaluating device 17 puts the deterioration evaluating process to anend.

When the evaluation result parameter SK is set in STEP89 through STEP91,and the deterioration evaluating process shown in FIG. 12 is ended, thedeterioration evaluating device 17 and the ECU 16 are turned off untilthe operation of the engine 1 is resumed. The value of the evaluationresult parameter SK is stored in the non-volatile memory such as anEEPROM or the like during the shutdown of the engine 1.

According to the deterioration evaluating process described above, whenthe predetermined period of time t/min has elapsed after the engine 1 isshut off, if the engine temperature TW (and the temperature near the HCadsorbent 7) drops to a temperature substantially equal to theatmospheric temperature TA (except if F/HCPG=0 or F/CND=0 or F/VROFF=0or F/HUMNG=1), the deteriorated state of the HC adsorbent 7 isevaluated.

In the above description, it is assumed that the engine temperature TWdrops to a temperature substantially equal to the atmospherictemperature TA when the predetermined period of time t/min has elapsedafter the engine 1 is shut off. However, since the manner in which theengine temperature TW drops after the engine 1 is shut off is affectedby the engine temperature TW and the atmospheric temperature TA at thetime the engine 1 is shut off, the engine temperature TW may notnecessarily be lowered to a temperature substantially equal to theatmospheric temperature TA when the predetermined period of time t/minhas elapsed, i.e., when the deterioration evaluating process shown inFIG. 12 is executed for the first time after the engine 1 is shut off,but it is possible that TW−TA≧DT in STEP86 shown in FIG. 12, e.g., asindicated by the curve d shown in FIG. 13. In such a case, thedeterioration evaluating device 17 increments the value of a countparameter C/DONE representing the number of times that the deteriorationevaluating process shown in FIG. 12 is executed, by “1” in STEP92, andthen compares the value of the count parameter C/DONE with apredetermined upper limit value N in STEP93. If C/DONE<N in STEP93, thenthe deterioration evaluating device 17 sets the set time of the offsettimer to a predetermined time Δt (see FIG. 13) in STEP94. Thereafter,the deterioration evaluating device 17 puts the deterioration evaluatingprocess shown in FIG. 12 to an end. At this time, the deteriorationevaluating device 17 and the ECU 16 are turned off. Therefore, afterelapse of the predetermined time Δt, the off timer is actuated toactivate the deterioration evaluating device 17 and the ECU 16, and thedeterioration evaluating device 17 executes the deterioration evaluatingprocess shown in FIG. 12. In the present embodiment, the predeterminedtime Δt is set to a time, e.g., 30 minutes, shorter than thepredetermined period of time t/min (two hours in the present embodiment)which determines the time for initially executing the deteriorationevaluating process shown in FIG. 12. However, the predetermined time Δtmay be the same as or longer than the predetermined period of timet/min. When the internal engine 1 is shut off, the count parameterC/DONE is initialized to “0”. While the engine 1 is being shut off, thecount parameter C/DONE is stored in the nonvolatile memory such as anEEPROM or the like.

If C/DONE>N in STEP93, i.e., if the engine temperature TW is notconverged to the atmospheric temperature TA when the deteriorationevaluating process shown in FIG. 12 is carried out as many times as theupper limit value N, then the deterioration evaluating device 17 regardsthis condition as inappropriate for finalizing the evaluation of thedeteriorated state of the HC adsorbent 7, and sets the evaluation resultparameter SK to “0” in STEP91. Thereafter, the deterioration evaluatingdevice 17 puts the deterioration evaluating process to an end. The upperlimit value N is set such that the period of time that elapses from theshutdown of the engine 1 to the time when the deterioration evaluatingprocess is carried out in an Nth cycle terminates short of the time whenthe steady humidity period Δtpx is finished, i.e., is equal to orshorter than the period t/max in FIG. 13.

When the operation of the engine 1 is resumed after the deterioratedstate of the HC adsorbent 7 is evaluated during the shutdown of theengine 1, the deterioration evaluating device 17 operates thedeterioration indicator 20 depending on the value of the evaluationresult parameter SK. Specifically, if the value of the evaluation resultparameter SK is “2”, i.e., if the deteriorated state of the HC adsorbent7 is the deterioration-in-progress state, then the deteriorationindicator 20 indicates the deterioration-in-progress state.

According to the above process, when the predetermined period of timet/min has elapsed after the engine 1 is shut off, if the enginetemperature TW (and the temperature near the HC adsorbent 7) does notdrop to a temperature substantially equal to the atmospheric temperatureTA, then the deterioration evaluating process shown in FIG. 12 issubsequently carried out in each predetermined time Δt until the enginetemperature TW drops to a temperature substantially equal to theatmospheric temperature TA.

According to the present embodiment, therefore, subsequently to the timewhen the predetermined period of time t/min has elapsed after the engine1 is shut off, when the engine temperature TW drops to a temperaturesubstantially equal to the atmospheric temperature TA, the deterioratedstate of the HC adsorbent 7 is actually evaluated, and the evaluationresult parameter SK is set. Inasmuch as the time t/max until the steadyhumidity period Δtpx in which the output voltage VRHUM of the humiditysensor 19 is substantially constant is finished after the engine 1 isshut off is relatively long (sufficiently longer than the predeterminedperiod of time t/min), there is basically no situation where the enginetemperature TW does not drop to a temperature substantially equal to theatmospheric temperature TA before the steady humidity period Δtpxexpires. Therefore, the deterioration evaluating process shown in FIG.12 is reliable in evaluating the deteriorated state of the HC adsorbent7 using the detected value of the output voltage VRHUM of the humiditysensor 19 within the steady humidity period Δtpx as a deteriorationevaluating parameter. Since the output voltage VRHUM of the humiditysensor 19 used to evaluate the deteriorated state of the HC adsorbent 7is detected while it is being substantially constant steadily, thedetected value of the output voltage VRHUM can be obtained withaccuracy. The deteriorated state of the HC adsorbent 7 is finally judgedby comparing the value which is produced by subtracting thecharacteristic change parameter VRHUMOFF corresponding to an offsetvoltage caused by the deterioration of the humidity sensor 19 from theoutput voltage VRHUM of the humidity sensor 19, with the thresholdVRHUM/JUD2. As a result, the evaluation of the deteriorated state of theHC adsorbent 7 can be performed highly reliably and accuratelyregardless of a characteristic change due to the deterioration of thehumidity sensor 19 and characteristic variations of individual units ofthe humidity sensor 19.

The humidity sensor 19 may be able to detect the relative humidity nearthe HC adsorbent 7 while the relative humidity is being substantiallyconstant. Therefore, the humidity sensor 19 is not required to be highlyresponsive, but may comprise a relatively inexpensive sensor.

In the present embodiment, if the value of the flag F/HCPG is “1”, i.e.,if it is recognized that the release of the HC and the moisture adsorbedby the HC adsorbent 7 during the preceding operation of the engine 1 iscompleted, then the deteriorated state of the HC adsorbent 7 isessentially evaluated. Stated otherwise, the deteriorated state of theHC adsorbent 7 is evaluated while the HC adsorbent 7 has adsorbed amaximum amount of moisture which it can adsorb in the deteriorated stateafter the engine 1 is shut off. In the present embodiment, furthermore,if the value of the flag F/CND is “1”, i.e., if the engine 1 issufficiently warmed up and the air-fuel mixture is stably combustedimmediately before the engine 1 is shut off, and also if the air-fuelratio KACT of the engine 1 has been kept close to the stoichiometricair-fuel ratio for the predetermined period of time, then thedeteriorated state of the HC adsorbent 7 is essentially evaluated.Stated otherwise, if the exhaust gas present around the HC adsorbent 7contains a sufficient amount of moisture and variations in the containedamount of moisture are small immediately after the engine 1 is shut off,then the deteriorated state of the HC adsorbent 7 is evaluated based onthe output voltage VRHUM of the humidity sensor 19 within the steadyhumidity period Δtpx. As a consequence, the deteriorated state of the HCadsorbent 7 is evaluated accurately and appropriately.

In the present embodiment, if the value of the characteristic changeparameter VRHUMOFF is excessively large, then the value of the flagF/HUMNG is set to “1”. In this case, the deteriorated state of the HCadsorbent 7 is not essentially evaluated. Therefore, the reliability ofthe evaluation of the deteriorated state of the HC adsorbent 7 ismaintained.

In the present embodiment, the humidity sensor 19 is disposed downstreamof the HC adsorbent 7. However, the humidity sensor 19 may be disposedupstream of the HC adsorbent 7 insofar as the humidity sensor 19 isdisposed near the HC adsorbent 7. In this case, the process (the processshown in FIG. 10) for acquiring the data of the characteristic changeparameter VRHUMOFF will be described supplementarily. Generally, thehumidity upstream of the HC adsorbent 7 changes from a relatively lowhumidity value quickly to a substantially constant high humidity value(substantially 100%) corresponding to the inherent humidity of theexhaust gas after the engine 1 has started to operate. That is, afterthe engine 1 has started to operate, the time required for the outputvoltage VRHUM of the humidity sensor 19 to reach a substantiallyconstant low level is shorter than with the curves a, b shown in FIG. 7.Therefore, if the humidity sensor 19 is disposed upstream of the HCadsorbent 7, the predetermined value TMVR/TSH2 relative to the engineoperation elapsed time TM used in STEP63 in FIG. 10 may be smaller thanthe value in the second embodiment.

In the second embodiment, the processing in STEP53 through STEP62 shownin FIG. 10 is performed in order to confirm that the output voltageVRHUM of the humidity sensor 19 has shifted from a high level to a lowlevel (to confirm the falling of the output voltage VRHUM) after theengine 1 has started to operate. According to the second embodiment, thedeteriorated state of the HC adsorbent 7 is evaluated using the outputvoltage VRHUM of the humidity sensor 19 while the engine 1 is not inoperation. Therefore, if the predetermined value TMVR/TSH2 relative tothe engine operation elapsed time TM used in STEP63 in FIG. 10 is set toa relatively large value, then the processing in STEP53 through STEP62shown in FIG. 10 may be dispensed with.

Furthermore, VRHUMOFF used in STEP88 shown in FIG. 12 according to thesecond embodiment may be replaced with the characteristic changeparameter VRHUMCH that is determined in STEP25 shown in FIG. 6 accordingto the first embodiment. In this case, however, the threshold VRHUM/JUD2used in STEP88 shown in FIG. 12 is different from that in the secondembodiment, and is basically smaller than the value determined from thedata table shown in FIG. 14.

A third embodiment of the present invention will be described below withreference to FIGS. 2, 3, and 5 and FIGS. 15 through 20. The presentembodiment differs from the first embodiment only as to a portion of theapparatus arrangement and the processing sequence of the deteriorationevaluating device. Therefore, those parts or functions of the thirdembodiment which are identical to those of the first embodiment aredenoted by reference characters and figures that are identical to thoseof the first embodiment, and will not be described in detail below.

According to the present embodiment, as shown in FIG. 15, the apparatushas, in addition to the humidity sensor 19 downstream of the HCadsorbent 7 of the exhaust gas purifier 8, a humidity sensor 25 upstreamof the HC adsorbent 7 near the HC adsorbent 7. An output voltage VFHUMof the humidity sensor 25 (an output voltage depending on the relativehumidity upstream of the HC adsorbent 7) is given, together with theoutput voltage VRHUM of the humidity sensor 19, to the deteriorationevaluating device 17. The upstream humidity sensor 25 is of the sametype as the downstream humidity sensor 19, and has the same outputcharacteristics (negative characteristics with respect to the relativehumidity) as the humidity sensor 19. In FIG. 2, the respective outputvoltages VRHUM, VFHUM of the humidity sensors 19, 25 are shown as havingthe same characteristics with respect to the relative humidity. However,the output characteristics of the humidity sensors 19, 25 do not need tobe fully identical to each other.

As shown in FIG. 3, the upstream humidity sensor 25 provided in thepresent embodiment is electrically connected to the deteriorationevaluating device 17 by two connectors 20 a, 20 b, as with thedownstream humidity sensor 19. The connector 20 a closer to the upstreamhumidity sensor 25 has a label resistive element 21 having a resistancedepending on the characteristics actually measured for each individualunit of the humidity sensor 25. With the upstream humidity sensor 25connected to the deterioration evaluating device 17, the deteriorationevaluating device 17 detects a resistance of the label resistive element21 relative to the inherent characteristics of the upstream humiditysensor 25 through the resistance detecting circuit 22, and sets thevalue of a parameter (to be described in detail later on) forcompensating for the effect of characteristic variations of individualunits of the upstream humidity sensor 25 based on the detectedresistance. The other structural details of the apparatus are identicalto those of the first embodiment. According to the present embodiment,the deterioration evaluating device 17 functions as an upstream changingtiming detecting means, a downstream changing timing detecting means, anintegrated moisture quantity data generating means, a characteristicchange detecting means, and a characteristic change compensating means.

Operation of the apparatus according to the present embodiment forevaluating the deteriorated state of the HC adsorbent 7 will bedescribed below.

According to the present embodiment, the deterioration evaluating device17 performs a processing sequence according to a flowchart shown in FIG.16 when the engine 1 starts to operate. In this processing sequence, theprocessing in STEP101 through STEP106 and the processing in STEP113 areidentical to the processing in STEP1 through STEP6 and the processing inSTEP10 shown in FIG. 4 according to the first embodiment. Therefore,these processing details will not be described below. As described abovewith respect to the first embodiment, for setting the value of the flagF/MCND, in addition to determining the conditions in STEP101 andSTEP103, the deterioration evaluating device 17 may determine whethersoaking has been completed or not, and the deterioration evaluatingdevice 17 may set to the value of the flag F/MCND to “0” if soaking isnot completed.

In step S107 following STEP106, the deterioration evaluating device 17sets the value of the parameter VRHUM/INI for compensating for theeffect of variations of the characteristics of individual units of thedownstream humidity sensor 19 in the process, to described later on, forevaluating the deteriorated state of the HC adsorbent 7, from thedetected data of the resistance LBLR of the label resistive element 21of the downstream humidity sensor 19 which has been acquired in STEP105,based on a predetermined data table or the like. The parameter VRHUM/INIhas been described above with respect to the first embodiment.Specifically, the parameter VRHUM/INI serves as a reference value forthe output voltage VRHUM of the downstream humidity sensor 19 if therelative humidity detected by the downstream humidity sensor 19 becomesa substantially constant high relative humidity (about 100%) after theengine 1 has started to operate, if the downstream humidity sensor 19 isbrand-new. According to the present embodiment, as with the secondembodiment, the value of the parameter VRHUM is used as representing thecharacteristics of individual units of the downstream humidity sensor19.

Then, the deterioration evaluating device 17 performs a processingsequence, which is the same as the processing in STEP105 through STEP107with respect to the downstream humidity sensor 19, with respect to theupstream humidity sensor 25, in STEP108 through STEP110. Specifically,in STEP108, the deterioration evaluating device 17 acquires the outputvoltage VFHUM of the upstream humidity sensor 25, and also acquires dataof the resistance LBLF of the label resistive element 21 relative to theupstream humidity sensor 25 through the resistance detecting circuit 22.In STEP109, the deterioration evaluating device 17 sets the presentvalue of the output voltage VFHUM of the upstream humidity sensor 25acquired in STEP108 as the initial value of a parameter VFHUM/MAXrepresentative of the latest value of a maximum value of the outputvoltage VFHUM of the upstream humidity sensor 25 and the initial valueof a preceding output parameter VFHUM/PRE representative of a precedingvalue of the output voltage VFHUM (a preceding value in each cycle timeof the processing sequence performed by the deterioration evaluatingdevice 17. In STEP110, the deterioration evaluating device 17 sets thevalue of the parameter VRHUM/INI for compensating for the effect ofvariations of the characteristics of individual units of the upstreamhumidity sensor 25 in the process, to described later on, for evaluatingthe deteriorated state of the HC adsorbent 7, from the detected data ofthe resistance LBLF of the label resistive element 21 of the upstreamhumidity sensor 25 which has been acquired in STEP108, based on apredetermined data table or the like. The parameter VFHUM/INI relativeto the upstream humidity sensor 25 serves as a reference value for theoutput voltage VFHUM of the upstream humidity sensor 25 if the relativehumidity detected by the upstream humidity sensor 25 becomes asubstantially constant high relative humidity (about 100%) after theengine 1 has started to operate, when the upstream humidity sensor 25 isbrand-new. According to the present embodiment, since the upstreamhumidity sensor 25 is of the same type as the downstream humidity sensor19, the value of the parameter VFHUM/INI relative to the upstreamhumidity sensor 25 is generally similar to the value of the parameterVRHUM/INI relative to the downstream humidity sensor 19, though theyslightly differ from each other depending on individual units of thehumidity sensors.

Then, the deterioration evaluating device 17 determines a deteriorationevaluating threshold TRSTMDT with which to determine whether the HCadsorbent 7 is in the deterioration-in-progress state or thenon-deteriorated state from the detected data of the engine temperatureTW (the initial engine temperature) of the engine 1 acquired in STEP102according to a predetermined data table as indicated by the broken-linecurve in FIG. 5 in STEP111. As described in detail later on, accordingto the present embodiment, the period of time that has elapsed from thestart of operation of the engine 1 is not used as a deteriorationevaluating parameter unlike the first embodiment, but the period of timethat has elapsed from a predetermining timing after the engine 1 hasstarted to operate is used as a deterioration evaluating parameter.Therefore, the deterioration evaluating threshold TRSTMDT according tothe present embodiment which is indicated by the broken-line curve inFIG. 5 is of a value slightly smaller than the deterioration evaluatingthreshold according to the first embodiment (which is indicated by thesolid-line curve in FIG. 5).

Then, the deterioration evaluating device 17 initializes, to “0”, thevalue of a timer TM (count-up timer) which measures a period of timethat has elapsed from the start of the operation of the engine 1, andalso initializes flags F/RST, F/FST, F/FSH to be described later on, to“0” in STEP112. Thereafter, the processing sequence shown in FIG. 16 isended.

After having carried out the processing sequence shown in FIG. 16 whenthe engine 1 starts to operate, the deterioration evaluating device 17carries out a processing sequence shown in FIGS. 17 and 18 in a givencycle time to evaluate the deteriorated state of the HC adsorbent 7while the engine 1 is idling immediately after the engine 1 has startedto operate.

Prior to specifically describing the processing sequence shown in FIGS.17 and 18, a basic concept of a process of evaluating the deterioratedstate of the HC adsorbent 7 according to the present embodiment willfirst be described below with reference to FIG. 19.

When the engine 1 starts operating, it emits an exhaust gas that issupplied through the exhaust system downstream of the engine 1 to the HCadsorbent 7, as described with respect to the first embodiment. At thistime, the relative humidity detected by the downstream humidity sensor19 changes as described with respect to the first embodiment.Specifically, immediately after the engine 1 has started to operate, therelative humidity detected by the downstream humidity sensor 19 is of alow level as indicated by the solid-line curve a or the broken-linecurve b in FIG. 19. When the adsorption of moisture in the exhaust gasby the HC adsorbent 7 is saturated, the relative humidity changes to atendency to increase monotonously from the low humidity level to a highhumidity level. The curves a, b in FIG. 19 are identical to the curvesa, b in FIG. 7 according to the first embodiment.

On the other hand, the relative humidity upstream of the HC adsorbent 7which is detected by the upstream humidity sensor 25 does not changeinstantaneously to a high humidity level due to the highly humid exhaustgas from the start of operation of the engine 1, but changes from a lowhumidity level to a high humidity level with a certain delay from thestart of operation of the engine 1 (see the curve c or d in FIG. 19).This is because it takes a slight period of time for the exhaust gasfrom the engine 1 to reach a location upstream of the HC adsorbent 7(the location of the upstream humidity sensor 25), the relative humidityaround the HC adsorbent 7 has been relatively low as the HC adsorbent 7has adsorbed moisture during the shutdown of the engine 1, and thecatalytic converter 6 upstream of the exhaust gas purifier 8 hasadsorbed humidity. Therefore, the output voltage VFHUM of the upstreamhumidity sensor 25 changes to a tendency to decrease monotonously from ahigh humidity level to a low humidity level with a certain delay fromthe start of operation of the engine 1 as indicated by the solid-linecurve c or the broken-line curve d in FIG. 19, for example. The curve cin FIG. 9 corresponds to data that is plotted when the upstream humiditysensor 25 is brand-new, and the curve d in FIG. 19 corresponds to datathat is plotted when the upstream humidity sensor 25 is deteriorated toa certain degree.

A changing timing (a time t1 or t2 in FIG. 19) at which the outputvoltage VFHUM of the upstream humidity sensor 25 changes to a tendencyto decrease monotonously from a high level to a low level is delayed asthe deterioration of the upstream humidity sensor 25 progresses, as withthe changing timing relative to the output voltage VRHUM of thedownstream humidity sensor 19 as described with respect to the firstembodiment.

A chanting timing at which the relative humidity detected by theupstream humidity sensor 25 changes to a tendency to increasemonotonously from a low humidity level to a high humidity level, or achanging timing at which the output voltage VFHUM of the upstreamhumidity sensor 25 changes to a tendency to decrease monotonously from ahigh level to a low level may suffer variations due to the moistureadsorption by the catalytic converter 6 even if the deterioration of theupstream humidity sensor 25 is constant. For example, the changingtiming at the time t1 in FIG. 19 may suffer variations even when theupstream humidity sensor 25 is brand-new.

If the changing timing of the relative humidity upstream of the HCadsorbent 7 which is detected by the upstream humidity sensor 25 is madeearlier, then the timing at which the HC adsorbent 7 starts to besupplied with an exhaust gas containing much moisture (the timing atwhich the HC adsorbent 7 starts to adsorb moisture) is also madeearlier. Therefore, the saturation of the adsorption of moisture by theHC adsorbent 7 is also made earlier, and hence the changing timing ofthe relative humidity downstream of the HC adsorbent 7 or the outputvoltage VRHUM of the downstream humidity sensor 19 is also made earlier.Conversely, if the changing timing of the relative humidity upstream ofthe HC adsorbent 7 is made later, then the changing timing of therelative humidity downstream of the HC adsorbent 7 or the output voltageVRHUM of the downstream humidity sensor 19 is also made later.

If the changing timing of the relative humidity upstream of the HCadsorbent 7 suffers variations as described above, then it is desirableto grasp the integrated amount of moisture supplied by the exhaust gasto the HC adsorbent 7 from the changing timing of the relative humidityupstream of the HC adsorbent 7 to the changing timing of the relativehumidity downstream of the HC adsorbent 7 for grasping the total amountof moisture adsorbed by the HC adsorbent 7 after the engine 1 hasstarted to operate. According to the present embodiment, not only thechanging timing of the output voltage VRHUM of the downstream humiditysensor 19 (hereinafter referred to as “downstream changing timing”), butalso the changing timing of the output voltage VFHUM of the upstreamhumidity sensor 25 (hereinafter referred to as “upstream changingtiming”) are detected, and integrated moisture quantity datarepresentative of the integrated amount of moisture that is supplied tothe HC adsorbent from the upstream changing timing to the downstreamchanging timing is determined as a deterioration evaluating parameterfor evaluating the deteriorated state of the HC adsorbent 7. In thepresent embodiment, the process of obtaining a deterioration evaluatingparameter is performed while the engine 1 is idling after the engine 1has started to operate, as with the first embodiment, and the engineoperation elapsed time from the upstream changing timing to thedownstream changing timing is obtained as integrated moisture quantitydata that serves as a deterioration evaluating parameter. Morespecifically, as shown in FIG. 19, it is assumed that the engineoperation elapsed time from the start of operation of the engine 1 tothe upstream changing timing (the time t1 relative to the curve c inFIG. 19 which corresponds to the data plotted when the upstream humiditysensor 25 is brand-new) is represented by TMF, and the engine operationelapsed time from the start of operation of the engine 1 to thedownstream changing timing (the time t3 relative to the curve a in FIG.19 which corresponds to the data plotted when the downstream humiditysensor 19 is brand-new) is represented by TMR (which is thedeterioration evaluating parameter in the first embodiment), and TMR−TMFis used as a deterioration evaluating parameter TMTRS/PM. In the presentembodiment, as with the first embodiment, the effect of thedeterioration of the humidity sensors 19, 25, and the effect ofcharacteristic variations of individual units of the humidity sensors19, 25 are compensated for. In addition, the downstream changing timingsignifies the adsorption saturation timing at which the adsorption ofmoisture by the HC adsorbent 7 is saturated, as with the firstembodiment. The upstream changing timing signifies the timing at whichthe HC adsorbent 7 starts to essentially adsorb moisture.

Based on the concept described above, the processing sequence accordingto the flowchart shown in FIGS. 17 and 18 will be described below. Thedeterioration evaluating device 17 carries out the processing sequenceshown in FIGS. 17 and 18 in a given cycle time after the engine 1 hasbeen activated. According to the processing sequence shown in FIGS. 17and 18, the deterioration evaluating device 17 determines the value ofthe flag F/MCND set in the processing sequence shown in FIG. 16 when theengine 1 starts to operate in STEP121. If F/MCND=0, then it means thatthe apparatus is in a state not suitable for evaluating the deterioratedstate of the HC adsorbent 7 or the present process of evaluating thedeteriorated state of the HC adsorbent 7 has already been finished.Therefore, the deterioration evaluating device 17 puts the processingsequence shown in FIGS. 17 and 18 to an end.

If F/MCND=1, then the deterioration evaluating device 17 increments thevalue of the engine operation elapsed time TM by a predetermined valueΔTM (fixed value) in STEP122, and then acquires the present detecteddata of the output voltage VFHUM of the upstream humidity sensor 25 andthe present detected data of the output voltage VRHUM of the downstreamhumidity sensor 19 in STEP123. Then, the deterioration evaluating device17 determines the value of the flag F/RST which is initialized by theprocessing sequence shown in FIG. 16 when the engine 1 starts tooperate, in STEP124. The flag F/RST has the same meaning as the flagF/RST used in the first embodiment, i.e., the flag F/RST is “1” when thedownstream changing timing relative to the downstream humidity sensor 19is detected, and “0” when the downstream changing timing is notdetected.

If F/RST=0 in STEP124, then the deterioration evaluating device 17performs the same processing as in STEP15 through STEP17 shown in FIG. 6according to the first embodiment in STEP125 through STEP127 to updatethe maximum output parameter VRHUM/MAX relative to the downstreamhumidity sensor 19 and update the value of the preceding outputparameter VRHUM/PRE. In this manner, after the engine 1 has started tooperate, the maximum value of the output voltage VRHUM of the downstreamhumidity sensor 19 is sequentially detected.

Then, the deterioration evaluating device 17 determines the value of aflag F/FST which is initialized by the processing sequence shown in FIG.16 when the engine 1 starts to operate, in STEP128. The flag F/FST is“1” when the upstream changing timing relative to the upstream humiditysensor 25 is detected, and “0” when the upstream changing timing is notdetected. If F/FST=0 in STEP128, then the deterioration evaluatingdevice 17 performs the same processing as in STEP125 through STEP127with respect to the upstream humidity sensor 25 in STEP129 throughSTEP131. Specifically, if the present value of the output voltage VFHUMof the upstream humidity sensor 25 (which is acquired in STEP123) isgreater than the maximum output parameter VFHUM/MAX relative to theupstream humidity sensor 25 (YES in STEP129), then the deteriorationevaluating device 17 updates the value of the maximum output parameterVFHUM/MAX with the present value of the output voltage VFHUM in STEP130.Regardless of the result of the decision in STEP129, the deteriorationevaluating device 17 updates the preceding output parameter VFHUM/PRErelative to the upstream humidity sensor 25 with the present value ofthe output voltage VFHUM in STEP131. According to the processing inSTEP129 through STEP131, the maximum value of the output voltage VFHUMof the upstream humidity sensor 25 is sequentially detected after theengine 1 has started to operate.

Then, the deterioration evaluating device 17 compares the present valueof the output voltage VFHUM of the upstream humidity sensor 25 with thevalue (VFHUM/MAX−VFHUM/JUD) which is produced by subtracting apredetermined value VFHUM/JUD (see FIG. 19) from the present value ofthe maximum output parameter VFHUM/MAX in STEP132. IfVFHUM≧VFHUM/MAX−VFHUM/JUD, then the deterioration evaluating device 17judges that the timing of the present cycle time is not the changingtiming at which the output voltage VFHUM of the upstream humidity sensor25 changes to the tendency to decrease monotonously (the timing at whichthe adsorption of moisture and HCs by the HC adsorbent 7 is essentiallystarted), and puts the present processing sequence shown in FIGS. 17 and18 to an end. The predetermined value VFHUM/JUD may be the same as thepredetermined value VRHUM/JUD (described above with respect to the firstembodiment) for detecting the downstream changing timing relative to thedownstream humidity sensor 19. However, in view of actual transitionalcharacteristics of the relative humidity upstream and downstream of theHC adsorbent 7, the predetermined values VFHUM/JUD, VRHUM/JUD may bedifferent from each other.

If VFHUM<VFHUM/MAX−VFHUM/JUD in STEP132, then the deteriorationevaluating device 17 judges that the timing of the present cycle time isthe upstream changing timing (the time t1 or t2 in FIG. 19), andcompares the present engine operation elapsed time TM with apredetermined value TM/SHF (fixed value) in STEP133. The predeterminedvalue TM/SHF is signified as an upper limit value for the engineoperation elapsed time TM at the normal upstream changing timing.Specifically, if the engine operation elapsed time TM at the upstreamchanging timing detected in STEP132 is in excess of TM/SHF, then theupstream humidity sensor 25 may possibly be suffering a failure. Thepredetermined value TM/SHF may be set for each individual unit of theupstream humidity sensor 25, as with the parameter TM/SH used in STEP19shown in FIG. 6 according to the first embodiment.

If TM<TM/SHF in STEP133 (in a normal case), then the deteriorationevaluating device 17 judges that the timing of the present cycle time isthe normal upstream changing timing relative to the upstream humiditysensor 25, and stores the present engine operation elapsed time TM asthe value of the upstream changing detecting parameter TMF (as the valueof the deterioration evaluating parameter TMTRS/PM for evaluating thedeteriorated state of the HC adsorbent 7) in STEP134. The deteriorationevaluating device 17 sets the value of the flag F/FST to “1” in STEP135,and puts the present processing sequence in FIGS. 17 and 18 to an end.In this case, F/FST=1 in STP128 from the next cycle time, and theprocessing from STEP136, to be described later on, is performed. Thedeterioration evaluating parameter TMTRS/PM is signified as representingthe integrated amount of moisture in the exhaust gas that is generatedby the engine 1 from the start of operation of the engine 1 until theupstream changing timing.

If TM≧TM/SHF in STEP133, then since the detected upstream changingtiming is excessively late and inappropriate, the deteriorationevaluating device 17 sets the flag F/MCND to “0” in STEP141, and putsthe present processing sequence in FIGS. 17 and 18 to an end. In thiscase, F/MCND=0 in STP121 from the next cycle time, and the processingsequence shown in FIGS. 17 and 18 are immediately put to an end.

In the processing sequence shown in FIGS. 17 and 18 after the upstreamdetecting parameter TMF has been obtained, F/FST=1 in the decisionprocessing in STEP128. In this case, the deterioration evaluating device17 performs the same processing as in STEP18 through STEP22 shown inFIG. 6 according to the first embodiment in STEP136 through STEP141, andputs the processing sequence shown in FIGS. 17 and 18 in the presentcycle time to an end. Specifically, the deterioration evaluating device17 compares the present value of the output voltage VRHUM of thedownstream humidity sensor 19 with the value (VRHUM/MAX−VRHUM/JUD) whichis produced by subtracting a predetermined value VRHUM/JUD from thepresent value of the maximum output parameter VRHUM/MAX in STEP136. IfVRHUM≧VRHUM/MAX−VRHUM/JUD, then the deterioration evaluating device 17judges that the timing of the present cycle time is not the changingtiming at which the output voltage VRHUM of the humidity sensor 19changes to the tendency to decrease monotonously (the timing at whichthe adsorption of moisture by the HC adsorbent 7 is saturated”), andputs the present processing sequence shown in FIGS. 17 and 18 to an end.The predetermined value VRHUM/JUD may be the same as the predeterminedvalue VRHUM/JUD used in the first embodiment.

If VRHUM<VRHUM/MAX−VRHUM/JUD in STEP136, then the deteriorationevaluating device 17 judges that the timing of the present cycle time isthe downstream changing timing (the time t3 or t4 in FIG. 19), andcompares the present engine operation elapsed time TM with apredetermined value TM/SHR (fixed value) in STEP137. The predeterminedvalue TM/SHR is signified as an upper limit value for the engineoperation elapsed time TM at the normal downstream changing timing, aswith the parameter TM/SH used in STEP19 shown in FIG. 6 according to thefirst embodiment. The predetermined value TM/SHR may be set for eachindividual unit of the downstream humidity sensor 19, as with theparameter TM/SH used in the first embodiment.

If TM<TM/SHR in STEP137, then the deterioration evaluating device 17judges that the timing of the present cycle time is the normaldownstream changing timing, and stores the present engine operationelapsed time TM as the value of the downstream changing detectingparameter TMR in STEP138. Then, the deterioration evaluating device 17determines a value (TMR−TMF) that is produced by subtracting the valueof the upstream changing detecting parameter TMF obtained in STEP134from the value of the downstream changing detecting parameter TMR,temporarily as a deterioration evaluating parameter TMTRS/PM in STEP139.Thereafter, the deterioration evaluating device 17 sets the value of theflag F/RST to “1” in STEP140, and puts the present processing sequencein FIGS. 17 and 18 to an end. The downstream changing detectingparameter TMR is signified as representing the integrated amount ofmoisture in the exhaust gas that is generated by the engine 1 from thestart of operation of the engine 1 until the downstream changing timing.When either one of the upstream humidity sensor 25 and the downstreamhumidity sensor 19 is brand-new, the deterioration evaluating parameterTMTRS/PM determined in STEP139 corresponds to the total amount ofmoisture that has actually been adsorbed by the HC adsorbent 7 after theengine 1 has started to operate presently.

If TM≧TM/SHR in STEP137, then since the detected downstream changingtiming is excessively late and inappropriate, the deteriorationevaluating device 17 sets the flag F/MCND to “0” in STEP141, and putsthe present processing sequence in FIGS. 17 and 18 to an end.

In the processing sequence shown in FIGS. 17 and 18 after the value ofthe downstream changing detecting parameter has been obtained, F/RST=1in the decision processing in STEP124. In this case, the deteriorationevaluating device 17 performs the processing from STEP141 shown in FIG.18. First, the deterioration evaluating device 17 determines the valueof a flag F/FSH which is initialized to “0” by the processing sequenceshown in FIG. 16 when the engine 1 starts to operate, in STEP142. Theflag F/FSH is “1” when a characteristic change parameter VFHUMOFF, to bedescribed later on, relative to the upstream humidity sensor 25, and acorrective quantity COR/TMTFS depending thereon have been determined,and “0” when such a characteristic change parameter VFHUMOFF and acorrective quantity COR/TMTFS have not been determined.

If F/FSH=1 in the decision processing in STEP142, then control goes tothe processing from STEP147 to be described later on. If F/FSH=0, thenthe deterioration evaluating device 17 compares the present engineoperation elapsed time TM with a predetermined value TMVF/TSH2 (see FIG.19) in STEP143. The predetermined value TMVF/TSH2 is a threshold fordetermining whether the relative humidity detected by the upstreamhumidity sensor 25 has reached a substantially constant high relativehumidity level (about 100%) after the upstream changing timing or not.The value of TMVF/TSH2 is experimentally determined in advance such thatif TM≧TMVF/TSH2, then the relative humidity detected by the upstreamhumidity sensor 25 reaches a substantially constant high relativehumidity level regardless of characteristic changes and characteristicvariations of individual units of the upstream humidity sensor 25.

If TM>TMVF/TSH2 in STEP143, then the deterioration evaluating device 17determines a value (=VFHUM−VFHUM/INI) which is produced by subtractingthe value of the parameter VFHUM/INI set depending on thecharacteristics of the individual unit of the upstream humidity sensor25 in STEP110 shown in FIG. 16 when the engine 1 starts to operate, fromthe present value of the output voltage VFHUM of the upstream humiditysensor 25, as a characteristic change parameter VFHUMOFF representativeof a characteristic change due to the deterioration of the upstreamhumidity sensor 25 in STEP144. As shown in FIG. 19, the characteristicchange parameter VFHUMOFF is signified as an offset voltage produced bythe deterioration of the upstream humidity sensor 25, as with thecharacteristic change parameter VRHUMOFF of the downstream humiditysensor 19 according to the second embodiment. If the upstream humiditysensor 25 is brand-new, then VFHUMOFF=0 regardless of characteristicvariations of individual units of the upstream humidity sensor 25. Asthe upstream humidity sensor 25 is progressively deteriorated, the valueof VFHUMOFF becomes larger. Therefore, the characteristic changeparameter VFHUMOFF represents the degree to which the humidity sensor 25is deteriorated regardless of characteristic variations of individualunits of the humidity sensor 25.

After having determined the characteristic change parameter VFHUMOFFrelative to the upstream humidity sensor 25, the deteriorationevaluating device 17 determines a corrective quantity COR/TMTFS forcorrecting the deterioration evaluating parameter TMTRS/PM determined inSTEP139 from the value of the characteristic change parameter VFHUMOFFin STEP145. The corrective quantity COR/TMTFS is determined from thevalue of the characteristic change parameter VFHUMOFF based on a datatable represented by the broken-line curve shown in FIG. 20, forexample. The corrective quantity COR/TMTFS corrects the deteriorationevaluating parameter TMTRS/PM in order to compensate for the effect of acharacteristic change due to the deterioration of the upstream humiditysensor 25. According to the data table shown in FIG. 20, the correctivequantity COR/TMTFS is determined such that when the value of thecharacteristic change parameter VFHUMOFF is sufficiently small (whenVFHUMOFF≦OFFX in FIG. 20), i.e., when the upstream humidity sensor 25 isbrand-new or nearly brand-new, COR/TMTFS=0, and when the value of thecharacteristic change parameter VFHUMOFF becomes large to a certainextent (VFHUMOFF>OFFX in FIG. 20), COR/TMTFS has a larger value as thecharacteristic change parameter VFHUMOFF is larger. As shown in FIG. 19,the corrective quantity COR/TMTFS is signified as correcting the valueof the upstream changing detecting parameter TMF obtained when acharacteristic change is being caused by the deterioration of theupstream humidity sensor 25 (the curve d in FIG. 19) into the value ofthe upstream changing detecting parameter TMF obtained when the upstreamhumidity sensor 25 is brand-new (the curve c in FIG. 19).

Having thus determined the corrective quantity COR/TMTFS relative to theupstream humidity sensor 25, the deterioration evaluating device 17 setsthe value of the flag F/FSH to “1” in STEP146, and thereafter performsthe processing from STEP147. In subsequent cycle times after the valueof the flag F/FSH has been set to “1” in STEP146, since the answer toSTEP142 is NO, the processing from STEP147 is immediately performedafter the decision processing in STEP142.

In STEP147, the deterioration evaluating device 17 compares the engineoperation elapsed time TM with a predetermined value TMVR/TSH2 (see FIG.19). The predetermined value TMVR/TSH2 is the same as the predeterminedvalue TMVR/TSH2 used in relation to the downstream humidity sensor 19according to the second embodiment. The value of TMVR/TSH2 isexperimentally determined such that if TM≧TMVR/TSH2, then the outputvoltage VRHUM of the downstream humidity sensor 19 reliably reaches asubstantially constant low voltage level after the downstream changingtiming regardless of the deteriorated state and characteristicvariations of the downstream humidity sensor 19.

If TM≧TMVR/TSH2 in STEP147, then the deterioration evaluating device 17determines a value (=VRHUM−VRHUM/INI) which is produced by subtractingthe value of the parameter VRHUM/INI set depending on thecharacteristics of the individual unit of the downstream humidity sensor19 in STEP107 shown in FIG. 16 when the engine 1 starts to operate, fromthe present value of the output voltage VRHUM of the downstream humiditysensor 19, as a characteristic change parameter VRHUMOFF representativeof a characteristic change due to the deterioration of the downstreamhumidity sensor 19 in STEP148. The characteristic change parameterVRHUMOFF is the same as the characteristic change parameter VRHUMOFF ofthe downstream humidity sensor 19 according to the second embodiment,and becomes larger as the deterioration of the downstream humiditysensor 19 progresses.

Then, the deterioration evaluating device 17 determines a correctivequantity COR/TMTRS for correcting the deterioration evaluating parameterTMTRS/PM from the value of the characteristic change parameter VRHUMOFFin STEP149. The corrective quantity COR/TMTRS is determined from thevalue of the characteristic change parameter VRHUMOFF based on a datatable represented by the solid-line curve shown in FIG. 20, for example.The corrective quantity COR/TMTRS corrects the deterioration evaluatingparameter TMTRS/PM in order to compensate for the effect of acharacteristic change due to the deterioration of the downstreamhumidity sensor 19. According to the data table shown in FIG. 20, aswith the corrective quantity COR/TMTFS relative to the upstream humiditysensor 25, the corrective quantity COR/TMTRS relative to the downstreamhumidity sensor 19 is determined such that when the value of thecharacteristic change parameter VRHUMOFF is sufficiently small (whenVRHUMOFF≦OFFX in FIG. 20), i.e., when the downstream humidity sensor 19is brand-new or nearly brand-new, COR/TMTRS=0, and when the value of thecharacteristic change parameter VRHUMOFF becomes large to a certainextent (VRHUMOFF>OFFX in FIG. 20), COR/TMTRS has a larger value as thecharacteristic change parameter VRHUMOFF is larger. As shown in FIG. 19,the corrective quantity COR/TMTRS is signified as correcting the valueof the downstream changing detecting parameter TMR obtained when acharacteristic change is being caused by the deterioration of thedownstream humidity sensor 19 (the curve b in FIG. 19) into the value ofthe downstream changing detecting parameter TMR obtained when thedownstream humidity sensor 19 is brand-new (the curve a in FIG. 19).

Then, the deterioration evaluating device 17 subtracts the difference(COR/TMTRS−COR/TMTFS) between the corrective quantities COR/TMTFS,COR/TMTRS determined respectively in STEP145, STEP149 from thedeterioration evaluating parameter TMTRS/PM determined in STEP139, thuscorrecting the deterioration evaluating parameter TMTRS/PM in STEP150.

The correction in STEP150 is equivalent to subtracting a value(=TMF−COR/TMTFS, see FIG. 19) that is produced by subtracting thecorrective quantity COR/TMTFS from the upstream changing detectingparameter TMF, from a value (=TMR−COR/TMTRS, see FIG. 19) that isproduced by subtracting the corrective quantity COR/TMTRS from thedownstream changing detecting parameter TMF obtained in STEP138 shown inFIG. 17. Therefore, the deterioration evaluating parameter TMTRS/PM thusobtained by being corrected in STEP150 depends on the ability of the HCadsorbent 7 to adsorb moisture, irrespective of characteristic changesdue to the deterioration of the upstream and downstream humidity sensors25, 19, characteristic variations of individual units of the humiditysensors 25, 19, and also variations in the changing timing of the actualrelative humidity at the location of the upstream humidity sensor 25.According to the present embodiment, as can be seen from FIG. 20, thecorrective quantities COR/TMTRS, COR/TMTFS relative to the humiditysensors 19, 25 are “0” when either one of the characteristic changeparameters VRHUMOFF, VFHUMOFF is smaller than the predetermined valueOFFX (when either one of detected characteristic changes of the humiditysensors 19, 25 is sufficiently small). Therefore, when VRHUMOFF<OFFX andVFHUMOFF<OFFX, the deterioration evaluating parameter TMTRS/PM is notvirtually corrected (is prohibited from being corrected).

Then, the deterioration evaluating device 17 performs the sameprocessing as the processing in STEP28 through STEP32 shown in FIG. 6according to the first embodiment in STEP151 through STEP155, finallyevaluating the deteriorated state of the HC adsorbent 7. Specifically,the deterioration evaluating device 17 compares the deteriorationevaluating parameter TMTRS/PM corrected in STEP150 with thedeterioration evaluating threshold TRSTMDT set in STEP111 shown in FIG.16 when the engine 1 starts to operate in STEP151, to determine whetherthe HC adsorbent 7 is in the non-deteriorated state in STEP152 or in thedeterioration-in-progress state in STEP153. If the HC adsorbent 7 is inthe deterioration-in-progress state, then the deterioration evaluatingdevice 17 operates the deterioration indicator 18 to indicate thedeterioration-in-progress state in STEP154. Then, the deteriorationevaluating device 17 resets the value of the flag F/MCND to “0” inSTEP155, and puts the processing sequence shown in FIGS. 17 and 18 to anend in the present operation of the engine 1. According to the presentembodiment, inasmuch as the start point for calculating thedeterioration evaluating parameter TMTRS/PM is the upstream changingtiming that is slightly later than the start of operation of the engine1, the deterioration evaluating threshold TRSTMDT is of a value slightlysmaller than with the first embodiment (the solid-line curve in FIG. 5)as indicated by the broken-line curve in FIG. 5.

According to the present embodiment, as described above, thedeteriorated state of the HC adsorbent 7 can be evaluated with accuracywhile compensating for the effect of characteristic changes due to thedeterioration of the upstream and downstream humidity sensors 25, 19,characteristic variations of individual units of the humidity sensors25, 19, and variations in the changing timing of the actual relativehumidity at the location of the upstream humidity sensor 25.

In the present embodiment, VFHUMOFF, VRHUMOFF are used as characteristicchange parameters relative to the upstream humidity sensor 25 and thedownstream humidity sensor 19. However, characteristic change parameterswhich are the same as the characteristic change parameters in the firstembodiment may be used. In such a case, the characteristic changeparameter that is used relative to the downstream humidity sensor 19 isidentical to the characteristic change parameter VRHUMCH (see FIG. 7) inthe first embodiment. The characteristic change parameter that is usedrelative to the upstream humidity sensor 25 may be the differencebetween the output voltage VFHUM of the upstream humidity sensor 25 atthe time when a predetermined period of time for the individual unit ofthe upstream humidity sensor 25 (which corresponds to the parameterTMVR/TSH relative to the downstream humidity sensor 19) has elapsed fromthe start of operation of the engine 1, and the parameter VFHUM/INIwhich serves as a reference value for the output voltage VFHUM of theupstream humidity sensor 25.

In the present embodiment, the deterioration evaluating parameterTMTRS/PM is corrected. However, the deterioration evaluating thresholdTRSTMDT, rather than the deterioration evaluating parameter TMTRS/PM,may be corrected. In such a case, the deterioration evaluating thresholdTRSTMDT may be corrected by adding the difference (COR/TMTRS−COR/TMTFS)between the corrective quantities COR/TMTRS, COR/TMTFS to thedeterioration evaluating threshold TRSTMDT, and the correcteddeterioration evaluating threshold TRSTMDT may be compared with thedeterioration evaluating parameter TMTRS/PM (which is obtained inSTEP139 shown in FIG. 17).

In the present embodiment, if either one of the values of thecharacteristic change parameters VRHU-MOFF, VFHUMOFF becomes greaterthan a suitable upper limit value (if it becomes excessively large),then the humidity sensors 19, 25 may possibly suffer a failure. In thiscase, the substantial evaluation of the deteriorated state of the HCadsorbent 7 (the processing from STEP150 shown in FIG. 18) may not beperformed.

In the present embodiment, the period of time (TMR−TMF) that has elapsedfrom the time when the upstream changing timing is detected to the timewhen the downstream changing timing is detected is used as a basic valueof the deterioration evaluating parameter TMTRS/PM. However, theintegrated value of the amount of fuel supplied in the period of timefrom the time when the upstream changing timing is detected to the timewhen the downstream changing timing is detected (which may be a commandvalue generated by the ECU 16), or the integrated value of a detected orestimated value of the amount of intake air of the engine 1 may be usedas a basic value of the deterioration evaluating parameter TMTRS/PM. Inthis case, the engine 1 may not be idling after it has started tooperate.

In the first through third embodiments, the present invention has beendescribed as being applied to a system for evaluating the deterioratedstate of the HC adsorbent 7 that is independently provided in theexhaust system of the engine 1. However, the present invention is alsoapplicable to the evaluation of a state such as a deteriorated state ofan HC adsorbent of a hydrocarbon adsorption catalyst which comprises acomposite combination of an HC adsorbent and a catalyst such as athree-way catalyst. The hydrocarbon adsorption catalyst comprises asupport of honeycomb structure whose surface is coated with zeolite asan HC adsorbent and which supports thereon a layer of precious metalsuch as platinum, palladium, rhodium, etc. as a constituent element of athree-way catalyst.

In the first through third embodiments, the exhaust gas purifier 8housing the HC adsorbent 7 therein has a structure shown in FIG. 1 orFIG. 15, for example. The exhaust gas purifier 8 may also be of astructure shown in FIG. 21.

The exhaust gas purifier 8 has two divided flow passages 28, 29 branchedfrom an upstream exhaust pipe 26, a substantially cylindrical housing 30communicating with a downstream portion of the divided flow passage 28,and a bypass exhaust pipe 31 (exhaust passage) housed concentrically inthe housing 30. The bypass exhaust pipe 31 is filled with a cylindricalHC adsorbent (hydrocarbon adsorbent) 32. The upstream exhaust pipe 26 isconnected to a downstream end of the catalytic converter 6 shown in FIG.1, for example.

A space 33 defined between the inner circumferential surface of thehousing 30 and the outer circumferential surface of the bypass exhaustpipe 31 serves as a cylindrical exhaust passage 33 into which theexhaust gas is introduced from the divided flow passage 28. The bypassexhaust pipe 31 has an upstream end (left end in FIG. 21) joined to thedownstream end of the divided flow passage 29 through an opening 30 adefined in the upstream end of the housing 30. The upstream end of thebypass exhaust pipe 31 has an outer circumferential surface sealinglyheld in close contact with the inner circumferential surface of theopening 30 a in the housing 30. The exhaust passage 33 in the housing 30is not in communication with the divided flow passage 29 at the opening30 a.

An EGR passage 34 (exhaust gas recirculation passage) extends from theupstream end of the bypass exhaust pipe 31. The EGR passage 34communicates with the bypass exhaust passage 31 through a communicationhole 35 defined in the circumferential wall of the upstream end of thebypass exhaust pipe 31. The EGR passage 34 is also connected to theintake pipe of the engine downstream of the throttle valve as with theEGR passage 13 according to the first through third embodiments. The EGRpassage 34 has an on/off valve (solenoid-operated valve) for opening andclosing the EGR passage 34.

The bypass exhaust pipe 31 has a downstream end (right end in FIG. 21)joined to a downstream exhaust pipe 27 through an opening 30 b definedin the downstream end of the housing 30. The downstream end of thebypass exhaust pipe 31 has an outer circumferential surface sealinglyheld in close contact with the inner circumferential surface of theopening 30 b in the housing 30. The bypass exhaust passage 31 in thehousing 30 is not in communication with the exhaust pipe 27 at theopening 30 b. The downstream end of the bypass exhaust pipe 31 has aplurality of communication holes 36 defined in its circumferential walland communicating with the exhaust passage 33 in the housing 30. Theexhaust passage 33 communicates with the bypass exhaust pipe 31 throughthe communication holes 36. The exhaust pipe 27 is vented to theatmosphere through another catalytic converter, a muffler, or the like.

A directional control valve 37 is disposed in a region where theupstream exhaust pipe 26 is branched into the divided flow passages 28,29 for selectively connecting the divided flow passages 28, 29 to theexhaust pipe 26. The directional control valve 37 is angularly movableabout a pivot shaft 38 selectively into a solid-line position and animaginary-line position by an actuator (not shown). When the directionalcontrol valve 37 is in the solid-line position, it disconnects thedivided flow passage 29 from the exhaust pipe 26 and connects thedivided flow passage 28 to the exhaust pipe 26. When the directionalcontrol valve 37 is in the imaginary-line position, it disconnects thedivided flow passage 28 from the exhaust pipe 26 and connects thedivided flow passage 29 to the exhaust pipe 26.

With the exhaust gas purifier 8 thus constructed, immediately after theengine 1 starts to operate, the directional control valve 37 is actuatedto the imaginary-line position. The exhaust gas supplied from the engine1 to the exhaust gas purifier 8 flows through the divided flow passage29, the bypass exhaust pipe 31 (including the HC adsorbent 32 housedtherein), and the exhaust pipe 27 into the atmosphere. At this time, HCscontained in the exhaust gas are adsorbed by the HC adsorbent 32 in thebypass exhaust pipe 31. When the directional control valve 37 isactuated to the solid-line position, the exhaust gas supplied from theengine 1 through the catalytic converter 6 (see FIG. 1) to the exhaustgas purifier 8 flows through the divided flow passage 28, the exhaustpassage 33 in the housing 30, the communication holes 36, and theexhaust pipe 27 into the atmosphere.

For evaluating the deteriorated state of the HC adsorbent 32 of theexhaust gas purifier 8, a humidity sensor 19 is provided downstream ofthe HC adsorbent 32 near the HC adsorbent 32, as shown in FIG. 21. Thedeteriorated state of the HC adsorbent 32 can be evaluated by thedeterioration evaluating device 17 which performs a processing sequencethat is exactly the same as the processing sequence in the firstembodiment or the second embodiment. Alternatively, in addition to thehumidity sensor 19, a humidity sensor 25 is provided upstream of the HCadsorbent 32 near the HC adsorbent 32, as shown in FIG. 21. In thiscase, the deteriorated state of the HC adsorbent 32 can be evaluated bythe deterioration evaluating device 17 which performs a processingsequence that is exactly the same as the processing sequence in thethird embodiment.

INDUSTRIAL APPLICABILITY

As described above, the present invention is useful as an apparatuscapable of appropriately monitoring, with an inexpensive arrangement, astate such as a deteriorated state of a hydrocarbon adsorbent providedin an exhaust system of an internal combustion engine that is used as apropulsive source or the like on automobiles, hybrid vehicles, andships.

1. An apparatus for monitoring a state of a hydrocarbon adsorbentdisposed in an exhaust passage of an internal combustion engine foradsorbing hydrocarbons in an exhaust gas emitted from the internalcombustion engine, using output data of a humidity sensor disposed nearthe hydrocarbon adsorbent, characterized by: characteristic changedetecting means for detecting a characteristic change of the humiditysensor based on output data of the humidity sensor under a predeterminedcondition, and characteristic change compensating means for correcting aparameter to grasp the state of the hydrocarbon adsorbent using theoutput data of the humidity sensor, based on the characteristic changedetected by said characteristic change detecting means.
 2. An apparatusfor monitoring the state of the hydrocarbon adsorbent according to claim1, characterized by: integrated moisture quantity data generating meansfor sequentially generating data of an integrated amount of moisturesupplied to said hydrocarbon adsorbent by the exhaust gas emitted fromthe internal combustion engine after the internal combustion engine hasstarted to operate, wherein said characteristic change detecting meansdetects the characteristic change of the humidity sensor based on achange due to the characteristic change of the humidity sensor, ofchanges of transitional characteristics of the output data of thehumidity sensor with respect to the data generated by said integratedmoisture quantity data generating means after the internal combustionengine has started to operate.
 3. An apparatus for monitoring the stateof the hydrocarbon adsorbent according to claim 2, characterized in thatsaid characteristic change detecting means detects the characteristicchange of the humidity sensor based on a change from a predeterminedreference value of characteristic change detecting output data whichcomprises the output data of the humidity sensor at the time when thedata generated by said integrated moisture quantity data generatingmeans has reached a predetermined value after the internal combustionengine has started to operate.
 4. An apparatus for monitoring the stateof the hydrocarbon adsorbent according to claim 1, characterized in thatsaid characteristic change detecting means detects the characteristicchange of the humidity sensor based on a change due to thecharacteristic change of the humidity sensor, of changes of transitionalcharacteristics of the output data of the humidity sensor with respectto a period of time that has elapsed after the internal combustionengine has started to operate.
 5. An apparatus for monitoring the stateof the hydrocarbon adsorbent according to claim 4, characterized in thatsaid characteristic change detecting means detects the characteristicchange of the humidity sensor based on a change from a predeterminedreference value of characteristic change detecting output data whichcomprises the output data of the humidity sensor at the time when theperiod of time that has elapsed after the internal combustion engine hasstarted to operate has reached a predetermined value.
 6. An apparatusfor monitoring the state of the hydrocarbon adsorbent according to claim2 or 3, characterized in that the state of said hydrocarbon adsorbent tobe monitored comprises a deteriorated state of said hydrocarbonadsorbent, and said humidity sensor is disposed downstream of saidhydrocarbon adsorbent, wherein a changing timing at which a humidityrepresented by the output data of said humidity sensor changes to atendency to increase monotonously from a low humidity level to a highhumidity level after the internal combustion engine has started tooperate is detected, and the data generated by said integrated moisturequantity data generating means at the detected changing timing is usedas said parameter for grasping the deteriorated state of saidhydrocarbon adsorbent.
 7. An apparatus for monitoring the state of thehydrocarbon adsorbent according to any one of claims 1, 4 and 5,characterized in that the state of said hydrocarbon adsorbent to bemonitored comprises a deteriorated state of said hydrocarbon adsorbent,and said humidity sensor is disposed downstream of said hydrocarbonadsorbent, further comprising: integrated moisture quantity datagenerating means for sequentially generating data of an integratedamount of moisture supplied to said hydrocarbon adsorbent by the exhaustgas emitted from the internal combustion engine after the internalcombustion engine has started to operate, and changing timing detectingmeans for detecting a changing timing at which a humidity represented bythe output data of said humidity sensor changes to a tendency toincrease monotonously from a low humidity level to a high humidity levelafter the internal combustion engine has started to operate, wherein thedata generated by said integrated moisture quantity data generatingmeans at the changing timing detected by said changing timing detectingmeans is used as said parameter for grasping the deteriorated state ofsaid hydrocarbon adsorbent.
 8. An apparatus for monitoring the state ofthe hydrocarbon adsorbent according to any one of claims 1 through 5,characterized in that the state of said hydrocarbon adsorbent to bemonitored comprises a deteriorated state of said hydrocarbon adsorbent,and the output data of said humidity sensor before a humidityrepresented by the output data of said humidity sensor is converged to ahumidity outside of said exhaust passage after said internal combustionengine has stopped operating is used as said parameter for grasping thedeteriorated state of said hydrocarbon adsorbent.
 9. An apparatus formonitoring the state of the hydrocarbon adsorbent according to claim 8,characterized in that the output data of said humidity sensor within aperiod of time in which the humidity represented by the output data ofsaid humidity sensor is maintained at a substantially constant levelafter said internal combustion engine has stopped operating is used assaid parameter for grasping the deteriorated state of said hydrocarbonadsorbent.
 10. An apparatus for monitoring the state of the hydrocarbonadsorbent according to claim 8, characterized in that the deterioratedstate of said hydrocarbon adsorbent is grasped based on said parameterafter said internal combustion engine has stopped operating under apredetermined condition.
 11. An apparatus for monitoring the state ofthe hydrocarbon adsorbent according to claim 1, characterized in thatsaid characteristic change compensating means corrects said parameterwhen the characteristic change of said humidity sensor which is detectedby said characteristic change detecting means exceeds a predeterminedquantity.
 12. An apparatus for monitoring the state of the hydrocarbonadsorbent according to claim 1, characterized in that the deterioratedstate of said hydrocarbon adsorbent is prohibited from being graspedbased on said parameter when the characteristic change of said humiditysensor which is detected by said characteristic change detecting meansexceeds a predetermined upper limit quantity.
 13. An apparatus formonitoring the state of the hydrocarbon adsorbent according to claim 1,characterized in that said humidity sensor has characteristic dataholding means for holding, in advance, data of characteristics of anindividual unit of said humidity sensor, and said characteristic changedetecting means detects the characteristic change of the humidity sensorbased on the output data of said humidity sensor and the data held bysaid characteristic data holding means.
 14. An apparatus for monitoringthe state of the hydrocarbon adsorbent according to claim 3,characterized in that said humidity sensor has characteristic dataholding means for holding, in advance, data specifying saidpredetermined value relative to the data generated by said integratedmoisture quantity data generating means, as the data of thecharacteristics of the individual unit of said humidity sensor, and saidcharacteristic change detecting means acquires the characteristic changedetecting output data of said humidity sensor using said predeterminedvalue which is specified by the data held by said characteristic dataholding means.
 15. An apparatus for monitoring the state of thehydrocarbon adsorbent according to claim 3 or 5, characterized in thatsaid humidity sensor has characteristic data holding means for holding,in advance, data specifying said reference value relative to saidcharacteristic change detecting output data as the data of thecharacteristics of the individual unit of said humidity sensor, and saidcharacteristic change detecting means acquires the characteristic changedetecting output data of said humidity sensor using said reference valuewhich is specified by the data held by said characteristic data holdingmeans.
 16. An apparatus for monitoring the state of the hydrocarbonadsorbent according to claim 13 or 14, characterized in that saidcharacteristic data holding means comprises a resistive element having aresistance depending on the value of the data of the characteristics ofthe individual unit of said humidity sensor.
 17. An apparatus formonitoring the state of the hydrocarbon adsorbent according to claim 15,characterized in that said characteristic data holding means comprises aresistive element having a resistance depending on the value of the dataof the characteristics of the individual unit of said humidity sensor.18. An apparatus for monitoring a state of a hydrocarbon adsorbentdisposed in an exhaust passage of an internal combustion engine foradsorbing hydrocarbons in an exhaust gas emitted from the internalcombustion engine, using output data of a plurality of humidity sensorsdisposed at different locations near the hydrocarbon adsorbent,characterized by: characteristic change detecting means for detectingcharacteristic changes of the humidity sensors based on output data ofthe respective humidity sensors under a predetermined condition, andcharacteristic change compensating means for correcting a parameter tograsp the state of the hydrocarbon adsorbent using the output data ofthe humidity sensors, based on the characteristic changes of thehumidity sensors detected by said characteristic change detecting means.19. An apparatus for monitoring the state of the hydrocarbon adsorbentaccording to claim 18, characterized by: integrated moisture quantitydata generating means for sequentially generating data of an integratedamount of moisture supplied to said hydrocarbon adsorbent by the exhaustgas emitted from the internal combustion engine after the internalcombustion engine has started to operate, wherein said characteristicchange detecting means detects the characteristic changes of thehumidity sensors based on a change due to the characteristic changes ofthe humidity sensors, of changes of transitional characteristics of theoutput data of the humidity sensors with respect to the data generatedby said integrated moisture quantity data generating means after theinternal combustion engine has started to operate.
 20. An apparatus formonitoring the state of the hydrocarbon adsorbent according to claim 19,characterized in that said characteristic change detecting means detectsthe characteristic changes of the humidity sensors based on a changefrom a predetermined reference value of characteristic change detectingoutput data which comprises the output data of the humidity sensors atthe time when the data generated by said integrated moisture quantitydata generating means has reached predetermined values for therespective humidity sensors after the internal combustion engine hasstarted to operate.
 21. An apparatus for monitoring the state of thehydrocarbon adsorbent according to claim 18, characterized in that saidcharacteristic change detecting means detects the characteristic changesof the humidity sensors based on a change due to the characteristicchanges of the humidity sensors, of changes of transitionalcharacteristics of the output data of the humidity sensors with respectto a period of time that has elapsed after the internal combustionengine has started to operate.
 22. An apparatus for monitoring the stateof the hydrocarbon adsorbent according to claim 21, characterized inthat said characteristic change detecting means detects thecharacteristic changes of the humidity sensors based on a change frompredetermined reference values of characteristic change detecting outputdata of the humidity sensors which comprises the output data of thehumidity sensors at the time when the period of time that has elapsedafter the internal combustion engine has started to operate has reachedpredetermined values for the respective humidity sensors.
 23. Anapparatus for monitoring the state of the hydrocarbon adsorbentaccording to claim 19 or 20, characterized in that the state of saidhydrocarbon adsorbent to be monitored comprises a deteriorated state ofsaid hydrocarbon adsorbent, and said humidity sensors comprise adownstream humidity sensor disposed downstream of said hydrocarbonadsorbent and an upstream humidity sensor disposed upstream of saidhydrocarbon adsorbent, further comprising: upstream changing timingdetecting means for detecting a changing timing at which a humidityrepresented by the output data of said upstream humidity sensor changesto a tendency to increase monotonously from a low humidity level to ahigh humidity level after the internal combustion engine has started tooperate, and downstream changing timing detecting means for detecting achanging timing at which a humidity represented by the output data ofsaid downstream humidity sensor changes to a tendency to increasemonotonously from a low humidity level to a high humidity level afterthe internal combustion engine has started to operate, wherein thedifference between the data generated by said integrated moisturequantity data generating means at the changing timing detected by saidupstream changing timing detecting means and the data generated by saidintegrated moisture quantity data generating means at the changingtiming detected by said downstream changing timing detecting means isused as said parameter for grasping the deteriorated state of saidhydrocarbon adsorbent.
 24. An apparatus for monitoring the state of thehydrocarbon adsorbent according to any one of claims 18, 21, and 22,characterized in that the state of said hydrocarbon adsorbent to bemonitored comprises a deteriorated state of said hydrocarbon adsorbent,and said humidity sensors comprise a downstream humidity sensor disposeddownstream of said hydrocarbon adsorbent and an upstream humidity sensordisposed upstream of said hydrocarbon adsorbent, further comprising:upstream changing timing detecting means for detecting a changing timingat which a humidity represented by the output data of said upstreamhumidity sensor changes to a tendency to increase monotonously from alow humidity level to a high humidity level after the internalcombustion engine has started to operate, downstream changing timingdetecting means for detecting a changing timing at which a humidityrepresented by the output data of said downstream humidity sensorchanges to a tendency to increase monotonously from a low humidity levelto a high humidity level after the internal combustion engine hasstarted to operate, and integrated moisture quantity data generatingmeans for generating data of an integrated amount of moisture suppliedto said hydrocarbon adsorbent by the exhaust gas emitted from theinternal combustion engine from the changing timing detected by saidupstream changing timing detecting means to the changing timing detectedby said downstream changing timing detecting means, wherein the datagenerated by said integrated moisture quantity data generating means isused as said parameter for grasping the deteriorated state of saidhydrocarbon adsorbent.
 25. An apparatus for monitoring the state of thehydrocarbon adsorbent according to claim 18, characterized in that saidcharacteristic change compensating means corrects said parameter wheneither one of the characteristic changes of said humidity sensors whichare detected by said characteristic change detecting means exceeds apredetermined quantity.
 26. An apparatus for monitoring the state of thehydrocarbon adsorbent according to claim 18, characterized in that saidcharacteristic change compensating means compares the characteristicchanges of the respective humidity sensors detected by saidcharacteristic change detecting means with a predetermined upper limitquantity, and prohibits the deteriorated state of said hydrocarbonadsorbent from being grasped based on said parameter when thecharacteristic change of at least one of said humidity sensors exceedssaid upper limit quantity.
 27. An apparatus for monitoring the state ofthe hydrocarbon adsorbent according to claim 18, characterized in thatsaid humidity sensors have respective characteristic data holding meansfor holding, in advance, data of characteristics of individual units ofsaid humidity sensors, and said characteristic change detecting meansdetects the characteristic changes of the humidity sensors based on theoutput data of said humidity sensors and the data held by saidcharacteristic data holding means.
 28. An apparatus for monitoring thestate of the hydrocarbon adsorbent according to claim 20 or 22,characterized in that said humidity sensors have respectivecharacteristic data holding means for holding, in advance, dataspecifying said reference values relative to said characteristic changedetecting output data of the humidity sensors as the data of thecharacteristics of the individual units of said humidity sensors, andsaid characteristic change detecting means acquires the characteristicchange detecting output data of said humidity sensors using saidreference values of the respective humidity sensors which are specifiedby the data held by said characteristic data holding means.
 29. Anapparatus for monitoring the state of the hydrocarbon adsorbentaccording to claim 27, characterized in that said characteristic dataholding means comprises resistive elements having resistances dependingon the value of the data of the characteristics of the individual unitsof said humidity sensors.
 30. An apparatus for monitoring the state ofthe hydrocarbon adsorbent according to claim 28, characterized in thatsaid characteristic data holding means comprises resistive elementshaving resistances depending on the value of the data of thecharacteristics of the individual units of said humidity sensors.
 31. Amethod of monitoring a state of a hydrocarbon adsorbent disposed in anexhaust passage of an internal combustion engine for adsorbinghydrocarbons in an exhaust gas emitted from the internal combustionengine, using output data of a humidity sensor disposed near thehydrocarbon adsorbent, comprising the steps of: detecting acharacteristic change of the humidity sensor based on output data of thehumidity sensor under a predetermined condition; and correcting aparameter to grasp the state of the hydrocarbon adsorbent using theoutput data of the humidity sensor, based on the detected characteristicchange.
 32. A method of monitoring a state of a hydrocarbon adsorbentdisposed in an exhaust passage of an internal combustion engine foradsorbing hydrocarbons in an exhaust gas emitted from the internalcombustion engine, using output data of a plurality of humidity sensorsdisposed at different locations near the hydrocarbon adsorbent,comprising the steps of: detecting characteristic changes of thehumidity sensors based on output data of the respective humidity sensorsunder a predetermined condition; and correcting a parameter to grasp thestate of the hydrocarbon absorbent using the output data of the humiditysensors, based on the characteristic changes of the humidity sensorsdetected in said detecting step.